LAN connectivity after power outage

I am using a cascade of three Fritz!Box routers to connect my solar power inverters to the internet. The main router acts as internet access and DHCP/DNS server. The intermediate router is connected as IP-Client to the main router via LAN and is used to operate a WiFi bridge to a detached building. In this buildig, the third router is coupled to the second router via WiFi and connects the solar power inverters via LAN. It acts as WiFi repeater of the intermediate router. Internet == Main =[LAN]= Intermediate =[WiFi]= Router 3 =[long LAN]= inverters provider Router Router In case of a temporary

Question by Axel Kemper on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.