Not registered on network...... not a signal issue

For several days possibly longer my phone has said not registered on network. No internet no phone. It sometimes works on WiFi which connects automatically at home and work so has taken a while to realise it's a real problem and not intermittent, but mostly says not registered if no where near a WiFi.
I have cleared the cache, taken the SIM out several times, restarted etc.
Mum on same network hers working fine sat next to me yesterday which is when I realised it was not a location thing but my phone or my SIM. Looking at the numerous other community posts I see it is not a phone thing but a SIM thing.
Having done everything I can to trick the phone into accepting it again, bar a full factory reset and loose all my data I am at a loss as to what to do now. Posh phone (galax S9) only a few months old rendered of less use than the ancient S3 I was using!
Please advise how to get the SIM reset so that I may call again. I'm paying for a service (internet when not at home and ability to make calls) that is not available to me.