Try these tips here Guide: Text Message Tips (not sent or received)
I don't know if this is relevant but my smsc is the correct number but it's in quotes and with ,145 at the end, if I try and change it to just the number and press update it says update error and pressing refresh brings it back to the one in quotes with the comma and 145 at the end. Using pixel 3 on Android 11.
We've always been told that it should be exactly formatted as shown:
Pay & Go message centre number is +447802092035
Pay Monthly message centre number is +447802000332
Have you tried removing the sim for 5 mins
More likely a network issue that's not been updated on the website yet @Kuribo but you could try getting a new sim from any O2 shop, they are free.