Unable to send messages in iMessage following a move from O2 to Lebara

I moved to Lebara last week from O2 and transferred my old number (on my iPhone 16). I now have have an issue in that I cannot send an SMS or iMessage. I have tried switching off, powering off and removing the SIM but issue still persists.

I have also tried:

1. Go to Settings > Messages and turn off iMessage.
2. Go to Settings > FaceTime and turn off FaceTime.
3. Restart your device
4. Turn iMessage and FaceTime back on.

And in addition, Settings -> Messages and enable iMessage. Choose ‘send & receive’ just below the enable button. Click on your apple id off the bottom of the list of contact methods and ‘sign out’ the iMessage account.

None of these have helped!

I can make and receive calls to my old number without issue and receive SMS messages (but not iMessages). I cannot send messages as I get the message that my “account is signed out of iMessage”.

I have checked on the Apple ID website and my old number is associated with my Apple ID. However, on my iPhone in the iMessage system, I can see the temporary Lebara number, but not my old number that has been ported across.

In Settings/My Number it is showing as the temporary Lebara number and I am unable to change that.

Please help!

Thank you,