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.
The DMZ for the ISPs router forward to the second router, then everything that hits your outside IP will be forwarded to router 2. Then on Router 2 you open the ports for your service and forward to the internal machine. That should all work fine.
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.
The actual cable modem can run in passthrough mode though. Look up the model and find the docs. Should be a quick and easy change, or your ISP at least should able to change it. It would be absurd if not.
Hey, I'm doing this now, using DuckDNS. But I had to forward a port thru Router 1, as you call it. That's your problem.
Maybe Tailscale would help you? (I've not used it though)
Or, instead of allowing port 80/443 traffic in, what I have is a random port used by my Wireguard VPN opened only. But I have to connect thru that when I'm not at home. I. E. only I can access my web server.
I'm actually behind 3 routers and still hosting stuff to the internet. My house is behind cgnat, I have two isp routers, which both connect to a pfsense router (ip of which is in the dmz of each isp router).
My pfsense router and a free vps hosted at oracle are both connected via tailscale. Pfsense router advertises specific subnet addresses to the tailnet. VPS uses caddy to reverse proxy to those subnet addresses to expose them to the internet.