21 devices in my house are connected to my UniFi wireless network; my macOS Mojave laptop, which was device #22 until last week, has stopped being able to connect. With more than 30 years of IT experience, I have absolutely no idea what to do about it

The worst thing: Occasionally it *does* connect, things work for a while, then crawl to a halt, then work again. I have no idea how to reproduce or even describe the issue to support except as “doesn’t work, most of the time”

Verify with a different network f.e. Office or freifunk to rule out a problem on the machine. Check system logs for messages like full ap and forced handover. Have you checked the UniFi controller health metrics for the specific client ?