How to install a Unifi UDM router on fiber internet when the gateway bridge mode is precluded by Xfinity's installation implementation.

I'm planning to install a Unifi Dream Machine (UDM) Pro as an alternate router and Wi-Fi device (when used with AP's). I currently have fiber internet to an ONU which outputs ethernet to an XB7 gateway. The obvious answer is to set the gateway to bridge mode then connect the UDM to one of the XB7 ethernet ports. However, since Xfinity documentation ("Use Bridge Mode on your wireless gateway") says that only one of the four XB7 switched ethernet ports can be used in bridge mode and that one port is permanently occupied by the WAN feed from the ONU, how has Xfinity provided an alternate setup for these customers to use their own Unifi equipment? Are there alternate gateways that get around this issue or some other fix that Xfinity has implemented?

There have to be hundreds of users out there that have this issue so I can't be the first one to ask this question. When I researched other community questions dealing with this issue, they all stop with the realization that without the ability to set the XB7 gateway into bridge mode, there is no answer stated that doesn't violate some networking standard (double NAT, etc.). I'm hoping there is an answer, but no one has taken the time to write it down so everyone can see it.