Update: There were two parts to this:
Initially, my network service provider did not correctly perform a sim swap to change my sim from physical sim to esim, but had instead add an esim to my account as they offer for people who want to use additional devices linked to their phones ie watches, etc.
That was step one to getting the issue resolved. Once I got them to properly do a sim swap on Wednesday this week, my smses also started coming through without issue.
Then there was iMessage - After sorting out the sim swap I then tried to activate iMessage and after switching it on I left it at that for 24 hours for the function to activate. It did not activate within the 24 hours. I then tried again and within a couple of hours the function was activated and is now working since yesterday afternoon.
So it does seems like a bit of a patience game provided the sim swap is done properly. iMessage needs sms functionality in order to activate, so if any option made for esim etc does not have the sms functionality active, iMessage will fail to activate.
If you have already sorted out the esim and smses work, then my suggestion is to select activate and leave it for atleast 24 hours. If it fails after 24 hours, try it again and leave it. Do not disable and enable repeatedly as iMessage takes time to activate.