подключил 504T
сначала работал нормально... потом DLS коннект есть, а инета нет

ставлю GDMT
Sep 8 12:00:10> Firewall NAT service started
Sep 8 12:00:11> Bridge Created: br0
Sep 8 12:00:11> Bridge Interface Added: eth0
Sep 8 12:00:12> pppd 2.4.1 started by root, uid 0
Sep 8 12:00:15> DSL Carrier is down
Sep 8 12:00:25> DSL Carrier is down
Sep 8 12:00:35> DSL Carrier is up
Sep 8 12:00:45> DSL Carrier is down
Sep 8 12:00:55> DSL Carrier is down
Sep 8 12:01:05> DSL Carrier is up

делаю принудительный коннект
Sep 8 12:04:32> Manual Conn State = 1
Sep 8 12:04:47> pppd 2.4.1 started by root, uid 0

и тишина

ставлю MMODE
Sep 8 12:07:16> Manual Conn State = 1
Sep 8 12:07:34> pppd 2.4.1 started by root, uid 0
Sep 8 12:07:34> Connect: ppp0 {--} nas0
Sep 8 12:07:34> Couldn't increase MTU to 1500.
Sep 8 12:07:34> Couldn't increase MRU to 1500

как подлечить?

У Вас как раз DSL и нет. Звоните провайдеру.
Да и прошивку обновить было бы неплохо.