Hi @AndyKH ,
I assume that other devices are able to connect to the 5 GHz SSID absolutely fine with exactly the same password/pre-shared key.
The first item which you've probably already tried is to forget the 5 GHz SSID from your phone and re-add it. The next likely item would be to step down the Encryption Type, if you are able to do this, purely for testing and then to restore to the previous. For example, I use "WPA2_Personal_AES" but could switch to the older and less secure "WPA_WPA2". The third thing to try is to change the 5 GHz SSID password to a slightly different value and this is probably the least likely to help.
I've seen symptoms similar to what you have described but with the Netgear MR1100 instead where certain devices seem only be able to connect to the 2.4 GHz SSID or the 5 GHz even though they can see both. Those devices are somewhat old and may not receive the latest updates which gives stepping down the encryption type a chance of helping.
Personally, I've found that connecting by 2.4 GHz works fine so I've considered the issue not worth debugging. If your measured bandwidth according to fast.com and speedtest.net is sufficient for your requirements, I would recommend you ignore the issue, or perhaps solve the issue with an upgraded mobile router in the future (e.g. a 5G one when the prices drop).
