Hi
I brought my son the iphone 12 pro for xmas and after discovering he can't send or receive picture messages we have done some research.Now I purchased his iphone with the esim at first we thought this was the issue.
So I had optus send me out a physical simcard which was 5G and still can't send pictures or receive.
Next was told to update apple software thats done , MMS is turned on like we were told to do.
So I decided to try my simcard out and put it in the iphone 12 now my simcard is 4G so I sent a picture message between my sons iphone 12 with my simcard to my other sons phone and it worked .
So now I have worked out if the simcard has 5G on it you won't be able to send or receive pictures messages until the network company come up with a solution.
My solution around this is I will be contacting optus again and asking if my son can have a 4g simcard then his problem is solved.
If you have access to a 4g simcard off a friend or family member that you can run your own test you will see :)
Hope this helps