Welcome to the community and sorry to hear about your difficulties with the new SIM. It does depend on the wording of the message coming up on your screen. If it’s referring to the SIM PIN and you're using one of our new 4G-ready SIM cards, it will require the PIN 7890.
I have the same issue, and the unlock PIN (7890) doesn't work either. I get a message saying 'SIM network unlock PIN is unsupported'. I'm bringing my number over from O2, and I was told yesterday that I have to switch SIMs today and it will work automatically. Now neither is working. Please help ASAP.