About three weeks ago my usually-reliable-af Unifi gear started having a problem, the wifi just turned to trash, the UI log was just filled with clients connecting and disconnecting over and over again. Everything was mostly just broken:
Most of my clients were showing up as "60%" on the wifi experience index thinger. Search turned up nothing useful so I'd thought I'd post this since I had to talk to Ubiquiti support, which escalated me to L2! I tried everything, a backup AP, reverting the controller version, trying all sorts of known-good firmware snapshots for both the USG-4-Pro and the access points. I even went to the grocery store and snagged a $30 AP just to get back on my feet.
Ends up, it was DNS(masq)! (Lol!) Somehow dnsmasq was disabled on my USG, so I turned it back on via the support instructions:
1. SSH into the USG and enter configuration mode, remember to turn on ssh first if you've not done that already!
2. Set the DHCP server to use Dnsmasq.
set service dhcp-server use-dnsmasq enable
3. Commit the changes and save the configuration.
commit ; save
And voila, a working network. I've been slowly adding clients back and so far so good!