I have a simIlar problem.
I transferred my mobile phone number from T-Mobile to BT several years ago. The BT SIM card has always worked OK in my Samsung Galaxy Core Prime G360F handset and the "my.bt.com" website has always displayed correct details for that phone and SIM.
Four weeks ago, I upgraded to a Samsung J6 handset. The old phone had a micro SIM card, so I had to order a nano SIM card, which I activated. Then, BT sent two APN configuration messages to my phone, with server details for the Internet and MMS. Having installed the configuration settings, I can make and receive phone calls and SMS messages. I can also use 3G and 4G for data, so everything seems to work OK. However, the "my.bt.com" website still shows details of my old phone and SIM card.
The old phone did not support Wi-Fi or 4G. The new one should be compatible. But as BT have not registered details of my new phone, the "my.bt.com" website still says my phone is not compatible. Furthermore, the BT Mobile app on the phone still states that I don't have access to 4G Calling or Wi-Fi calling. That doesn't seem to be factually correct.
To test the function, I checked that Wi-Fi calling is enabled in the phone settings. Then, I actived flight mode (to prevent the handset from making or receiving ordinary mobile phone calls via a local mast). Then, I re-enabled Wi-FI, and called my landline number. The call connected OK, which confirms that Wi-Fi Calling is available and works. Sadly, I have no way of testing 4G Calling.
Unfortunately, I cannot use the previously suggested solution, because my old phone requires a micro SIM card, whilst the new phone uses a nano SIM card.
Three on-line chats with BT technical support staff (based in India) produced no assistance whatsoever, apart from an unhelful suggestion that if I want to use Wi-Fi calling, I will need to upgrade my phone! Do you have any better suggestions, please?