5GHz did work for about 5 minutes a few nights ago, the first time I plugged the BT TV box in via ethernet. When I realised I disconnected the ethernet to try a wireless connection which instantly made the 5GHz drop out and hasn't appeared since.
I did try all 3 hubs in the same location, however, to make sure there wasn't just a bad spot in the hallway I moved it to the top of the stairs with a 10m ethernet. I had the same issue as expected. I guess, although very unlikely, I could have had 3 bad hubs. I will ring and get a new one to test, I may also purchase a 3rd party one to see if that makes a difference. For now, I am going to take the hub to my parents and see if it works on 5GHz at their house.
I will update the thread when I test the hub at my parents and if/when I get a new one or a 3rd party one.
Took the easy option and bought myself a TP Link Archer AX73. The new hub connects to 2.4GHz, 5GHz and 6GHz perfectly. There is obviously no interference from anywhere or the 5GHz wouldn't work on the new one. I dont know how, but I imagine I just got incredibly unlucky with 3 SH2's.
Thanks everyone who offered a solution, I appreciate you all taking time to try and help.