Thanks all. I am in the UK and had already checked to see if I could configure the regulatory domain on my Kobo via "normal" means and couldn't see anything. The AP and the Kobo are about 2m apart in the same room so that's probably not it. I'll check the config file on the Kobo. The AP allows me to set the channel on 2.4 GHz but not on 5 GHz, which seems really odd. I am surprised whenever I see people using 120, 124 or 128 because they seem more trouble than they are worth.
I have used
http://wifinigel.blogspot.com/2018/0...se-exempt.html as my source of 5 GHz info. I hope it is still currrent.