Xfinity XB8/XLE-WN failover with Ubiquiti UniFi network behind UniFi Cloud Gateway (UCG) Max

UPDATE (sparing you from reading everything below) -- I figured it out. My original failover test was bad. After finding and reading https://www.xfinity.com/support/articles/storm-ready-lights it turns out my original test showed a blinking white lite on the XLE-WN, not solid, meaning there was a problem. I misinterpreted this as being in cellular failover mode. I think I ran the test too soon after activation, and things just weren't ready. I repeated the test a few minutes ago, and failover worked. The XB8 was blinking amber, the XLE was solid white, and the internet was up, obviously at the slower speeds but it remained funtional.

Original Post below...

I just switched from Comcast Business to Xfinity (residential). With Comcast Business (CB) I had a CB gateway and a cellular failover gateway. The two gateways were connected via a CAT-6 cable so the cellular gateway could detect if the primary gateway failed.

My entire home network is Ubiquiti UniFi -- UCG Max as the main gateway, and then additional UniFi switches and wireless access points. The CB gateway was configured in Bridge Mode. I had the main CB gateway connected to the 2.5 Gbps port (port 5) of the UCG Max, configured as primary internet. The cellular failover was connected to port 1 of the UCG Max. If the primary (coax/broadband) ever went down, the cellular failover came online, the UCG Max detected the outage, and automatically failed over to the cellular backup. It worked perfectly.

The new hardware I have is the Xfinity XB8 gateway with "Storm Ready" XLE-WN wifi extender/cellular backup.

First order of business is that I couldn't use Bridge Mode. Not a huge deal; I just had to set up DMZ and then define a bunch of my internal DNS names since the XB8 doesn't support "hairpinning" (accessing an internal machine by its public DNS name).

The problem is the failover. The coax is connected to the XB8, and then port 5 of the UCG Max is connected to port 4 (the "red") port of the XB8. This works fine. I then activated the XLE-WN, and it activated fine. I then did a failover test -- disconnected the coax from the XB8. The light on the XB8 started blinking amber, and the XLE-WN started blinking white. This indicated the system was in failover mode. However, none of my devices, wired or wireless, could connect to the internet. The failover wasn't working.

I should clarify that I am NOT using the Xfinity-provided wifi; everything is connected through UniFi, either wired or wireless, but since UniFi (the UCG Max) is connected to the XB8, shouldn't it maintain connectivity if the XLE-WN is working?

I read that you can set up wired backhaul on the XLE-WN, but you have to connect a CAT-6 cable between the red ports on both the XB8 and XLE-WN. My UCG Max is plugged in to the red port on the XB8.

Documentation on this seems limited at best, and I even asked ChatGPT and Claude and neither were able to provide a workable solution.