Hello,
The user @wharbauer has recently had issues with WireGuard and FritzBox as well, and has made a short example in the comments section of how everything should be set up. You can find the thread here: TRB140 and Static Route with WireGuard VPN
The configuration shouldn’t be any different when using a RUT956 for this.
If this is not something you’re looking for, could you please provide a topology of your desired configuration?
Regards,
M.