@Jb2020 and others, I had the same issue and was able to fix it. The odd thing is that everything showed properly when I first setup the xFi device and activated it. Things went south when I called into xFinity to see why my speed maxes out at 550 Mb/sec, instead of the 1 Gb / sec for which I am paying.
When the automated assistant answered, it said I needed to activate my device (the second activation). I had no choice by to agree. Once I did, all of my wireless devices successfully connected, but xFi only showed my wired devices connected and none of my wireless ones.
After calling into xFinity support and spending two and a half hours, I couldn't take any more. (I swear they pass you around, when they are out of ideas, instead of transferring you to a higher-level tech.
Anyways . . . . .
I think I found the issue -- at least it worked on my system. Come on xFinity, you should be on top of this. Also, why is the setting I mention below changing. To add more details, I also factory reset the modem with no help . . .
HERE IS WHAT WORKED FOR ME:
(Note, I did this only for the 2.4 GHz Band)
Once the UI came back, all of my devices showed in the list. The odd thing is that the setting reverted from 20/40 back to 20. (xFi came set to 20/40, from what I remember and to what I have had previous routers set)
I'm betting there is a bug in the UI, which is causing this issue. Along these same lines, I cannot turn-off the MoCA setting, even though it was initially turned-off when I received my modem.
**** UPDATE ****
I had to do the same with the 5 GHz Band too. I just changed the setting from 20 to 20/40/60/80 and saved.