Finally got to speak to someone with access to the Hub systems. Admitted that the chat team just have access to the sales systems which is not sufficient in this case. Took my IMEI off me manually and hopefully this will be manually updated onto the system within 5 working days because the usual automated update (from reinserting the SIM) clearly isn’t working in my case.
Finally got My BT to recognise the iPhone X. Swapped the nano SIM card out into an old Nokia N78 using a SIM adapter and the network was triggered to send out new settings. Then My BT recognised the N78 & IMEI.
Then returned the SIM to the iPhone X and its Type & IMEI was recognised. This hadn't worked before, so the manual change by the hub team must have unfrozen the ability to recognise new phones.
Only problem left now is the unaccessible settings - will see if they unfreeze at my next bill date.
Finally, and the icing on the cake.... My BT app is showing Wi-Fi Calling & 4G Calling options!! Although neither is working just yet. These weren't there before the network recognised the phone type, which explains why I was so keen to get this problem fixed. The network needs to know the phone before it'll offer you these functions!!