@Disconnected Have you followed the procedure here: https://community.o2.co.uk/t5/How-to-Guides/SIM-Swap-a-mini-guide/ba-p/1060675
If you're still trying to use your old sim, make sure you've put your old sim into your new phone correctly. Have you turned the phone off and back on again?
The only option you have is to contact customer service to request a replacement SIM.
Online methods won't work because they send a verification text to the broken SIM card.
Contact details here Guide: How to find help & contact O2
Track usage, upgrade and get support with the My O2 appDownload My O2