@Foxyfox8511: Bit of a puzzling one. With 5G/LTE/3G/2G (auto connect) enabled, the phone will search for the strongest available signal in your area. If there are no 5G service in your area, and O2 have confirmed this, then attempting to connect to 5G might produce a weaker overall Mobile Data signal (although this is admittedly a complete guess). I'd recommend getting clarity from O2 about which regions currently support 5G though, so you can take advantage of the speeds if you're ever in one of the designated areas.