Wrong number being used for iMessage (old new number)

I had to get a prepaid sim (with a new phone number that I had to activate) to have my old number ported to me which has now happened.

iMessage will now only receive and send from my “new” number (which I don’t want to use) and I can’t change it to my old number.

I’ve tried turning iMessage off and restarting my phone but no luck. There “new” number is greyed out and ticked and my old number is not there to be selected.