For the record, I ended up looking into this in a bit more detail, and I’ve put some ideas on Wifi together in another post here.
In this post I argue against the idea that 5GHz channels above 112 are in some way unsuitable for Fairphones (referred to in an earlier post in this thread). On the contrary, mine is now happily connected on channel 120 with a link speed of 433 Mbps.
It turned out, in my case, that the problem apparently had nothing to do with channel selection. It was to do with security settings. All my devices have now moved to 5GHz.
@LudwigStecher it’s possible this is your problem too, though if you aren’t able to check the router settings yourself, you might talk to the admin about it.