I don’t really understand the problem here. Do these routers each have their own internet connection? Why can’t you just attach whatever device you are using to host stuff to one of them, configure your router for port forwarding, and be done? To get a domain name for free, you can use https://www.noip.com/.
If that mysteriously doesn’t work, you might want to investigate if your internet provider uses CGNAT (mine does). In that case, you might be able to contact them so they’ll turn it off for you. I don’t know about Germany, but in Austria they have to comply with your request, by law.
If you can’t do that or don’t want to expose your device to the internet directly, you have other options depending on whether you want your stuff to be public or not. For private services setting up WireGuard using wg-quick (on your Hetzner server) is really easy, reliable, and very secure. For public stuff, you might want to look at one of the services listed here. I recommend Cloudflare Tunnel, though it’s only meant for web stuff, no gameservers etc.
Feel free to ask for more help if you need more details.
Oh my god this is awesome!! I wanted to make something like this for myself for a while but never got around to it, unfortunately.