SIM activation and lost password

Hi, I totally understand the measures that I D have taken, and if I can't resolve my problem now, then so be it. I got a replacement SIM last Friday for a lost phone. I cannot remember my password to log in, so when trying to activate the SIM, I indicated I hadn't set up an account yet. However I've tried numerous times to then enter the SIM number and i t responds saying there's an error. I'm not sure I can resolve this online as I can't reset my password either as the security code will only go to my lost phone! Grateful for any help 😁