На проработавшем в непрерывном режиме более 10 дней и без каких-либо сбоев 500T внезапно погасли все индикаторы кроме "Power", через несколько минут работа возобновилась, но все настройки оказались сброшенными.

Sep 8 12:00:06> Valid Configuration Tree
Sep 8 12:00:10> Firewall NAT service started
Sep 8 12:00:12> Bridge Created: br0
Sep 8 12:00:12> Bridge Interface Added: eth0
Sep 8 12:00:13> pppd 2.4.1 started by root, uid 0
Sep 8 12:00:13> Before PADI send PADT to mac:0:48:136:0:102:158, session id:9e22
Sep 8 12:00:15> DSL Carrier is training
Sep 8 12:00:25> DSL Carrier is up
Sep 8 12:00:35> DSL Carrier is training
Sep 8 12:00:45> DSL Carrier is up
Sep 8 12:01:53> System Call Error
Sep 8 12:01:53> Connecting PPPoE socket: 00:00:00:00:00:00 0000 0x1000d3b8
Sep 8 12:01:53> Couldn't get channel number: Transport endpoint is not connected
Sep 8 12:01:53> Doing disconnect
Sep 8 12:01:53> pppd 2.4.1 started by root, uid 0
Sep 8 12:02:35> DSL Carrier is down
Sep 8 12:02:55> DSL Carrier is up
Sep 8 12:03:05> DSL Carrier is down
Sep 8 12:03:15> DSL Carrier is training
Sep 8 12:03:25> DSL Carrier is up
Sep 8 12:03:34> Connecting PPPoE socket: 00:00:00:00:00:00 0000 0x1000d3b8
Sep 8 12:03:34> Couldn't get channel number: Transport endpoint is not connected
Sep 8 12:03:34> Doing disconnect
Sep 8 12:03:34> System Call Error
Sep 8 12:03:39> pppd 2.4.1 started by root, uid 0
Sep 8 12:04:57> Got group error |Bad username/password|
Sep 8 12:04:57> Got group error |Bad username/password|
Sep 8 12:05:19> System Call Error
Sep 8 12:05:19> Connecting PPPoE socket: 00:00:00:00:00:00 0000 0x1000d3b8
Sep 8 12:05:20> Couldn't get channel number: Transport endpoint is not connected
Sep 8 12:05:20> Doing disconnect
Sep 8 12:05:20> pppd 2.4.1 started by root, uid 0
Sep 8 12:05:39> Got group error |Bad username/password|
Sep 8 12:07:00> System Call Error
Sep 8 12:07:00> Connecting PPPoE socket: 00:00:00:00:00:00 0000 0x1000d3b8
Sep 8 12:07:00> Couldn't get channel number: Transport endpoint is not connected
Sep 8 12:07:00> Doing disconnect
Sep 8 12:07:00> pppd 2.4.1 started by root, uid 0
Sep 8 12:08:41> Connecting PPPoE socket: 00:00:00:00:00:00 0000 0x1000d3b8
Sep 8 12:08:41> Couldn't get channel number: Transport endpoint is not connected
Sep 8 12:08:41> Doing disconnect
Sep 8 12:08:41> System Call Error
Sep 8 12:08:46> pppd 2.4.1 started by root, uid 0
Sep 8 12:09:15> DSL Carrier is down
Sep 8 12:09:47> pppd 2.4.1 started by root, uid 0
Sep 8 12:10:24> pppd 2.4.1 started by root, uid 0

Отключение скорее всего связано с пропаданием электропитания
Сброс настроек вызван тем, что Вы их не сохранили (Tools-> System Commands->Save all)

Очень жаль, что больше нет возможности редактировать свои записи.

Итак, 6 часов, полет нормальный. Подключение произошло как обычно (10 секунд мигает индикатор ADSL > 5 секунд горит постоянно > 10 секунд опять мигает > наконец-то устанавливает соединение):

Jan 25 05:31:51> pppd 2.4.1 started by root, uid 0
Jan 25 05:33:32> System Call Error
Jan 25 05:33:32> Connecting PPPoE socket: 00:00:00:00:00:00 0000 0x1000d3b8
Jan 25 05:33:32> Couldn't get channel number: Transport endpoint is not connected
Jan 25 05:33:32> Doing disconnect
Jan 25 05:33:32> pppd 2.4.1 started by root, uid 0
Jan 25 05:35:12> System Call Error
Jan 25 05:35:12> Connecting PPPoE socket: 00:00:00:00:00:00 0000 0x1000d3b8
Jan 25 05:35:12> Couldn't get channel number: Transport endpoint is not connected
Jan 25 05:35:12> Doing disconnect
Jan 25 05:35:12> pppd 2.4.1 started by root, uid 0
Jan 25 05:36:53> System Call Error
Jan 25 05:36:53> Connecting PPPoE socket: 00:00:00:00:00:00 0000 0x1000d3b8
Jan 25 05:36:53> Couldn't get channel number: Transport endpoint is not connected
Jan 25 05:36:53> Doing disconnect
Jan 25 05:36:53> pppd 2.4.1 started by root, uid 0
Jan 25 05:38:33> System Call Error
Jan 25 05:38:33> Connecting PPPoE socket: 00:00:00:00:00:00 0000 0x1000d3b8
Jan 25 05:38:33> Couldn't get channel number: Transport endpoint is not connected
Jan 25 05:38:33> Doing disconnect
Jan 25 05:38:34> pppd 2.4.1 started by root, uid 0
Jan 25 05:40:14> System Call Error
Jan 25 05:40:14> Connecting PPPoE socket: 00:00:00:00:00:00 0000 0x1000d3b8
Jan 25 05:40:14> Couldn't get channel number: Transport endpoint is not connected
Jan 25 05:40:14> Doing disconnect
Jan 25 05:40:14> pppd 2.4.1 started by root, uid 0
Jan 25 05:41:55> System Call Error
Jan 25 05:41:55> Connecting PPPoE socket: 00:00:00:00:00:00 0000 0x1000d3b8
Jan 25 05:41:55> Couldn't get channel number: Transport endpoint is not connected
Jan 25 05:41:55> Doing disconnect
Jan 25 05:41:55> pppd 2.4.1 started by root, uid 0
Jan 25 05:43:35> System Call Error
Jan 25 05:43:35> Connecting PPPoE socket: 00:00:00:00:00:00 0000 0x1000d3b8
Jan 25 05:43:35> Couldn't get channel number: Transport endpoint is not connected
Jan 25 05:43:35> Doing disconnect
Jan 25 05:43:36> pppd 2.4.1 started by root, uid 0
Jan 25 05:45:16> System Call Error
Jan 25 05:45:16> Connecting PPPoE socket: 00:00:00:00:00:00 0000 0x1000d3b8
Jan 25 05:45:16> Couldn't get channel number: Transport endpoint is not connected
Jan 25 05:45:16> Doing disconnect
Jan 25 05:45:16> pppd 2.4.1 started by root, uid 0
Jan 25 05:46:57> System Call Error
Jan 25 05:46:57> Connecting PPPoE socket: 00:00:00:00:00:00 0000 0x1000d3b8
Jan 25 05:46:57> Couldn't get channel number: Transport endpoint is not connected
Jan 25 05:46:57> Doing disconnect
Jan 25 05:46:57> pppd 2.4.1 started by root, uid 0
Jan 25 05:48:37> System Call Error
Jan 25 05:48:37> Connecting PPPoE socket: 00:00:00:00:00:00 0000 0x1000d3b8
Jan 25 05:48:37> Couldn't get channel number: Transport endpoint is not connected
Jan 25 05:48:37> Doing disconnect
Jan 25 05:48:38> pppd 2.4.1 started by root, uid 0
Jan 25 05:50:18> System Call Error
Jan 25 05:50:18> Connecting PPPoE socket: 00:00:00:00:00:00 0000 0x1000d3b8
Jan 25 05:50:18> Couldn't get channel number: Transport endpoint is not connected
Jan 25 05:50:18> Doing disconnect
Jan 25 05:50:18> pppd 2.4.1 started by root, uid 0
Jan 25 05:51:59> System Call Error
Jan 25 05:51:59> Connecting PPPoE socket: 00:00:00:00:00:00 0000 0x1000d3b8
Jan 25 05:51:59> Couldn't get channel number: Transport endpoint is not connected
Jan 25 05:51:59> Doing disconnect
Jan 25 05:51:59> pppd 2.4.1 started by root, uid 0
Jan 25 05:53:39> System Call Error
Jan 25 05:53:39> Connecting PPPoE socket: 00:00:00:00:00:00 0000 0x1000d3b8
Jan 25 05:53:39> Couldn't get channel number: Transport endpoint is not connected
Jan 25 05:53:39> Doing disconnect
Jan 25 05:53:39> pppd 2.4.1 started by root, uid 0
Jan 25 05:55:20> System Call Error
Jan 25 05:55:20> Connecting PPPoE socket: 00:00:00:00:00:00 0000 0x1000d3b8
Jan 25 05:55:20> Couldn't get channel number: Transport endpoint is not connected
Jan 25 05:55:20> Doing disconnect
Jan 25 05:55:20> pppd 2.4.1 started by root, uid 0
Jan 25 05:57:01> System Call Error
Jan 25 05:57:01> Connecting PPPoE socket: 00:00:00:00:00:00 0000 0x1000d3b8
Jan 25 05:57:01> Couldn't get channel number: Transport endpoint is not connected
Jan 25 05:57:01> Doing disconnect
Jan 25 05:57:01> pppd 2.4.1 started by root, uid 0
Jan 25 05:58:41> System Call Error
Jan 25 05:58:41> Connecting PPPoE socket: 00:00:00:00:00:00 0000 0x1000d3b8
Jan 25 05:58:41> Couldn't get channel number: Transport endpoint is not connected
Jan 25 05:58:41> Doing disconnect
Jan 25 05:58:41> pppd 2.4.1 started by root, uid 0
Jan 25 06:00:22> System Call Error
Jan 25 06:00:22> Connecting PPPoE socket: 00:00:00:00:00:00 0000 0x1000d3b8
Jan 25 06:00:22> Couldn't get channel number: Transport endpoint is not connected
Jan 25 06:00:22> Doing disconnect
Jan 25 06:00:22> pppd 2.4.1 started by root, uid 0
Jan 25 06:02:02> System Call Error
Jan 25 06:02:02> Connecting PPPoE socket: 00:00:00:00:00:00 0000 0x1000d3b8
Jan 25 06:02:02> Couldn't get channel number: Transport endpoint is not connected
Jan 25 06:02:02> Doing disconnect
Jan 25 06:02:03> pppd 2.4.1 started by root, uid 0
Jan 25 06:03:43> System Call Error
Jan 25 06:03:43> Connecting PPPoE socket: 00:00:00:00:00:00 0000 0x1000d3b8
Jan 25 06:03:43> Couldn't get channel number: Transport endpoint is not connected
Jan 25 06:03:43> Doing disconnect
Jan 25 06:03:43> pppd 2.4.1 started by root, uid 0
Jan 25 06:05:24> System Call Error
Jan 25 06:05:24> Connecting PPPoE socket: 00:00:00:00:00:00 0000 0x1000d3b8
Jan 25 06:05:24> Couldn't get channel number: Transport endpoint is not connected
Jan 25 06:05:24> Doing disconnect
Jan 25 06:05:24> pppd 2.4.1 started by root, uid 0
Jan 25 06:07:04> System Call Error
Jan 25 06:07:04> Connecting PPPoE socket: 00:00:00:00:00:00 0000 0x1000d3b8
Jan 25 06:07:04> Couldn't get channel number: Transport endpoint is not connected
Jan 25 06:07:04> Doing disconnect
Jan 25 06:07:05> pppd 2.4.1 started by root, uid 0
Jan 25 06:08:45> System Call Error
Jan 25 06:08:45> Connecting PPPoE socket: 00:00:00:00:00:00 0000 0x1000d3b8
Jan 25 06:08:45> Couldn't get channel number: Transport endpoint is not connected
Jan 25 06:08:45> Doing disconnect
Jan 25 06:08:45> pppd 2.4.1 started by root, uid 0
Jan 25 06:09:45> DSL Carrier is training
Jan 25 06:09:57> Tag error: TAG_HOST_UNIQ
Jan 25 06:09:57> Failed to negotiate PPPoE connection: 25
Jan 25 06:09:57> System Call Error
Jan 25 06:10:02> pppd 2.4.1 started by root, uid 0
Jan 25 06:10:05> DSL Carrier is up
Jan 25 06:10:15> DSL Carrier is training
Jan 25 06:10:25> DSL Carrier is up
Jan 25 06:10:33> Connecting PPPoE socket: 00:30:88:00:66:9e fe27 nas0 0x1000d3b8
Jan 25 06:10:33> Connect: ppp0 {--} nas0
Jan 25 06:10:34> PPPoE Connect with IP Address xx.xxx.xx.x
Jan 25 06:10:34> PPPoE Connection Successfully Established
Jan 25 06:10:34> PPPoE Connect with Gateway IP Address: xx.xxx.xx.x
Jan 25 06:10:34> WAN IP address xx.xxx.xx.x
Jan 25 06:10:34> WAN gateway xx.xxx.xx.x
Jan 25 06:10:34> primary DNS address xx.xxx.xx.x
Jan 25 06:10:34> secondary DNS address xx.xxx.xx.x

Может быть, причиной перезагрузки модема была какая-нибудь дос-атака доморощенных кулхацкеров?
Во всяком случае, может быть логи пригодятся...

Необходимые кнопки были нажаты сразу после перепрошивки и настройки.
А в момент перезагрузки модема я находился в 50см от него и все видел с самого начала :
загрузка сайтов с 3-й попытки > прекращение связи > погасли все индикаторы кроме Power > загорелись индикаторы с невозможностью соединения. Кроме того UPS даже не пискнул, предупреждая о нештатной ситуации