I don't understand Russian at all...Any TP-LINK local technical support in your region? Try to contact them.

In notes of the latest firmware, it didn't mention that you cannot downgrade. In this case, it should not cause any damage. If you doubt it is firmware issue, give a try.

no-ip

There is a newer firmware than yours. Give a try.

Are you trying to connect WDR4300 to the main router by cable and use it as an access point, or connect them wirelessly and use it in WDS mode? For the former---access point,...

WDR7500 and C7 are two different products. Why you buy a Chinese product? Chinese products cannot be upgraded to English version... If you don't understand Chinese at all, it is absolutely a bad...