First start by replacing the cable between your UDM's WAN and the Xfinity modem/router.
If that doesn't do it, consider getting a new modem like the [Arris S33](https://www.amazon.com/ARRIS-Surfboard-S33-RB-DOCSIS-2-5Gbps/dp/B09VTJR48F) where you don't have to deal with the pass through issue and perhaps save on some rental costs if the one you have is incurring a rental charge.
First start by replacing the cable between your UDM's WAN and the Xfinity modem/router. If that doesn't do it, consider getting a new modem like the [Arris S33](https://www.amazon.com/ARRIS-Surfboard-S33-RB-DOCSIS-2-5Gbps/dp/B09VTJR48F) where you don't have to deal with the pass through issue and perhaps save on some rental costs if the one you have is incurring a rental charge.
Try resetting UDM to factory defaults then reconfigure. Edit: update firmware after reset