Hi,
problem solved!
Since Fritzbox as the internet gateway is very common at least in germany, I like to refer to that thread which finally solved my issue to help those of you who found this thread first:
RUT950-Wireguard-with-Fritzbox
another very helpful thread within the recherche:
How to get info that WireGuard (site-to-site) is down?
Thanks
/Mav