iPhone 14 Pro only sending messages via text and not via Message?

Why is my iPhone 14Pro only sending messages via text and not via iMessage?

I had a new SIM card fitted two days ago and today I used my normal group iMessage but it wasn’t delivered so it had to be sent via text message (Green) and replies only came to me not the group.

I checked in Settings and it is set correctly, switched the phone off and on but still no iMessage available.

Can anyone advise what to do next?

Thanks