You will have been given a new log in for the new account. Sounds like you are logging into an old one.
Check the new emails received relating to new contract for log in details.
If you need help resetting your password you can do that as part of the new log in process or call CS to reset it for you.
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion.
This is not Customer Service so we are unable to help with account specific issues.Currently using:
Pixel 7a (O2 & Lyca), One Plus 6 (Sfr), iPhone 12 Pro Max (Vodafone)