So the UDM SE with an ALL-BM410 bridge from Allnet works perfectly on a Green.ch Fiber Anschluss.
The only thing that cost me nerves was getting the Swisscom registration page on the screen. This didn’t happen automatically and only when I reconnected to the WLAN, even though I was connected to the LAN the whole time.
But once I figured it out, it worked.
After registration there was an error message from the Swisscom registration page, but about 15 minutes later my Anschluss was up and running and has been working perfectly and without any interruptions since then.
I’m extremely satisfied.
Many thanks to everyone for your support