At least cause WiFi works at half duplex. 54Mbit/s is a channel speed but real can be much lower. Ideally 27Mbit/s but interference, distance, maintenance traffic etc affect it that is why 24-25Mbit/s "G" is a quite good result.

Is not any chance from router settings to speed up?

No chance. 50Mbit/s can be achieved on 802.11n not G. You need new router and perhaps new WiFi adapter (if your support only B/G)

Mihail821

Thank you very much for help!

Don't you also know any solution for the problem I've posted in the first post?

As СергейП said, the only solution is to reset settings and then configure all settings manually (not from backup/restore) and that process is too hard for me, because configuration is too complicated.

It`s only solution to make factory reset (traditionally for D-Link after upgrading FW make factory reset). I know about few opensource firmwares where is no need to do it (Padavan firmwares and Wive-NG-RTNL by sfstudio) except very rare situation and you can use old config after reset. Also I know special utilities to make config migration easier (for example in Merlin firmware you also sometimes need to reset router but you can easilly use old config and even restore JFFS partition with all your certificates, scripts etc by using special script). So you have to write down all setting and make reset and I also suppose more good firmware for your router is Vampik edition of firmware maked by "Enthusiasts" and Openwrt (if you plan to use some new 3G/4G modem).

When I've made FW update, it made no config reset and all settings were maintained. But now I can't change admin password, because in 1.00 version I had changed default username and in 1.22 version that change is not possible and it always should be "admin".

Is not any script to force password change the router? From firebug, for example. Or is not possible to correct *.bin configuration file and change password here?