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.
This right here. Since you can’t really configure the ISP router (1), DMZ is the way to go. The DMZ feature on home routers usually allows you to select a specific device or devices to add to the DMZ so make sure to reserve/set a static IP on your router (2) and configure the DMZ on your ISP router (1) to include only that specific IP. DMZ essentially forwards all ports to that device.