setting new phone on old number with the same provider

[adr14n]

I have a Tescomobile Samsung j5 2017 on contract that had finished last year and I think we went onto pay as you go still on the SIM it originally came with. This weekend my screen broke and I bought a Samsung A21s from tescomobile online sim only. I have set it up with the sim from my old phone and it appears to be sending and receiving texts and calls. However I cannot access my samsung account through the phone. I enter my ID and password but the two step authentication fails because the phone initiated a ne phone number to the SIM card. The phone says it has sent a text with a number to verfy but this never arrives. The same happens with Google and Whatsapp text verification

How do I get the old number back onto the SIM card? Tesco suggested changing the number in contacts but this did not allow the authentication. Do I need to request a PAC number even though I am continuing with the same provider?

[Shazzkid]

The phone can't initiate a new number on your sim. Either youre using a new sim and therefore a new number, or you're using the old one.

[adr14n]

ok so follow up is that I cannot find a way to verify my phone account. Thank you Samsung! 1 I have a new sim with my old number registered to the new phone but i still cannot get into my Samsung account from anywhere. not the phone or the wbsit. it does not eve send verification emails, not even on the account Iam using now. I have all my backed up info on a system I cant log into. All I get is repeated timeouts on verifications as the number or emails dont come through. Live chat is too busy and I cant call during the day as wife is using the phone. Aarrŕghhh