help setting up xfinity gateway and Orbi router in bridge mode

I run my xFi in Bridge mode.  There are a couple of gotchas I have found in switching things around.  The main thing is the IP address used to access the modem.  I don't know what it defaults to, but it is best to set it yourself to avoid trouble.  I set my xFi IP address to 10.0.0.1 so that the Orbi network uses 192.168.1.x.  If you don't set that in the xFi gateway before entering bridge mode, you cannot change it without leaving bridge mode and you don't want both the xFi and Orbi trying to use the same IP address range or the Orbi picking some random IP range.  I have a bunch of static IP addresses assigned to a lot of my devices, so I want the Orbi to use that range from 192.168.1.x.

Once you have that set, you can enable Bridge mode on the Gateway "At a Glance" page:

Enabling bridge mode turns off the xFi WIFI radios and makes it ready to connect an Orbi WIFI system to it.  The LAN settings and WIFI will no longer be accessible:

As mentioned, you may not need to do this, but it's not difficult. If you get stuck and unable to connect to either the Orbi or xFi over your network while you are in between routers, you can plug a computer directly into them to configure them as long as you know the IP address and make sure the connected computer has an address in the same network range, for example, in my case I'd set the connecting computer to 10.0.0.100 or 192.168.1.100 depending on if connecting to xFi or Orbi directly.