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.
Passthrough in this sense just means that the ISP cable modem only acts as a modem, handing off all traffic to your router to control. Essentially it just disabled NAT so you won’t have double-NAT’ing happening. It’s a standard setting on all cable modems, so I know that part is possible, but it’s more about getting your ISP to enable that if they have it locked.