I struggled to find a way to tell Messages to use my original number. I could not follow the steps set out in the iD Web site - maybe they were for a different messaging up. I did eventually find an Advanced setting for phone number which was blank so I entered the correct number and restarted the phone. Messages remained stuck with the interim number from the new SIM.
I ended up replacing Messages with a different messaging app - might have been Messenger? - and that worked. Then I re-installed Messages and nominated it as my default SMS app and at last it’s as I had it before. I’m a novice really, but there must be other folks who would struggle even more!