AC86U - Need troubleshooting ideas

Hi - I've been having odd issues with my AC86U mesh setup, and I'm not even sure where to start troubleshooting or searching for how to fix the problem, so I wanted to see if anyone had any suggestions. (I'm a noob and know just enough to be dangerous, so apologies if I'm missing something obvious or not providing enough info below.)

I've got Xfinity's xFI gigabit modem (I used to use my own Netgear, but after a series of problems with bringing my own equipment, I gave up) with two AC86U...

AC86U - Need troubleshooting ideas