Thank you for your repsonse.
I've found this topic, but it doesn't seem to state a solution.
The thing is that I want my Swisscom-box to connect to a Wireguard instance that is on a Raspberry Pi on another network.
So:
Swisscom-Anschluss -> Internet -> Other ISP -> Raspberry Pi with Wireguard.
At the moment I connect directly to Wireguard with my phone and laptop, but I want to route all my traffic through the Swisscom-box to the Wireguard instance.