Yippie-a-yeah we have fibre optic network now 
but therefore we don’t have a public IP anymore. 
That’s no longer offered in Germany - or if it is, only at extra cost and for business customers only, not private households
That’s why I need a different solution. Ideally, a “dummy-proof” one.
will VPN work perhaps?
Depends on the ISP. Telekom still does as far as I know. But others don’t. In any case, Roon has been offering this solution for a while:
Had the same issue. If both your ISP at home and your phone contract allow for unrestricted IPv6 connectivity, this might be working without a static IP4.
In my case activating IPv6 on the server was mandatory, as well as deactivating Upnp in the router, instead manually opening ports solely for IPv6.
After activating IPv6, check Settings > Roon ARC > Advanced and see if roon is transmitting a valid IPv6 to the internet.