For the very first time i tried to selfhost a static site (lighttpd + duckdns) but I’ve failed… Is it even possible to selfhost a static site from “router 2” while i’m behind two routers? “Router 1” is from ISP and i have no possibility to change any settings except subnet, standard gateway, dhcp on/off and DMZ. -> no port forwarding. "Router 2 is Asus with port-forwarding available and i’m using an OpenVPN configuration on it (if that matters). DuckDNS could see the external ip from R2, but i couldn’t access it.
You don’t want two routers as that creates a double NAT
Setup a service and them install Tailscale/Netbird on your devices. The reason double NAT is bad is that it can break NAT traversal used to allow you to directly remote access a device away from home.