TOTOLINK A7000R with OpenWrt as extended network

I am kinda new to network world. Recently bought TOTOLINK A7000R but router was still "losing" network and needed to be restarted what was annoying. I instlalled a OpenWrt software and have a little problem. I have a main router in other room with ip: and from this ruter I have a cable connected to a Totolink. It is working but the devices connected to totolink keep getting different subnet and totolink itself has an ip: Evrey time I want to change LAN interface to e.g. I am getting errors and rollback.

I was looking for a tutorial for that over internet but almost everywere is says to make a bridge from wireless to wireless. I want to connect from main router to a Totolink via cable (WAN port or LAN) to spreading network thru totolink.