@lduboeuf
Well, I didn't try to reset the APN, I am scared that nothing works afterwards.
I will try the terminal command proposed by Wally (I hope this won't have bad effects on SMS) and if nothing changes, maybe I will try to reset the APN.
I won't try a reinstallation, I am not expert enough of UBports (that's why I bought this second-hand smartphone with UT already installed).
If I cannot succeed in fixing the problem, well, my daughter will live without MMS (but it's a pity because this is our only way to exchange pictures during the week when she is on boarding school).