If you could force a way to do strictly 5GHz bands, that’d be amazing. Currently, forcing it to AC seems to not do anything for the advanced settings unless your router does strictly single-band broadcasting. When it is broadcasting both 5GHz and 2.4GHz, it seems to not want to connect to the 5GHz, causing the connection speed to be slower.
Are you using it with PiFi Kit/USB adapter (assuming so if wireless pairing)?
There was perviously an option in Settings to disable dual-band scan i.e to force it to 5Ghz only (the app can get that back but it was just if you select 5Ghz from available networks then it should be 5Ghz so forcing it seemed unnecessary)
When you said doesn’t want to connect to the 5Ghz - when you go to WiFi in the app and it shows available networks, is it not pairing with it?