Hello there, Dave.
I'm not aware of any issues that prevents iOS 18 users from connecting to the network. I'd recommend testing your SIM in another phone just to see if there's anything else going on. If the SIM is working in another phone, it would point to your updated phone being the issue. If it's still not working in another phone, it might be an account related issue, or a signal related issue. Let us know how you get on.
Pete.
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