Re: the 5GHz channel selection. Its because away from the router there are a series of strong 5GHz channels that are blocked from the router but not from the rooms in which I can receive the 5GHz signal. Hence the router thinks these channels are clear when in fact they aren't in the places where the signal is being used.
Also smart channel switching on current equipment is generally really poor (or even with enterprise level equipment):
'Setting the channel to “auto” should similarly be avoided, in spite of how tempting it is. The issue is that the Unifi APs don’t work with each other when they auto-select channels, and this frequently leads to a situation in which your access points (and your neighbors’ APs, too) constantly stampede across the spectrum, chasing an ever-moving chunk of unused air and constantly interfering with each other. Manual channel selection stakes a claim on some spectrum for each AP, and it should result in more consistent connectivity and a more evenly distributed use of the available spectrum (especially if your neighbors keep their stuff set on auto).'
From
https://arstechnica.com/information-technology/2018/07/enterprise-wi-fi-at-home-part-two-reflecting-...I can test with another router (a BT HH 5) as even as I was typing this on my phone the router restarted! Happy days....
