I have problems with sms on my S9 on Mint as well. However, I have found that if I am in an area with strong T-Mobile (Mint) coverage, the sms and mms problems do not appear. As soon as I lose a couple of bars of signal strength, the phone has difficulty sending/receiving both sms and mms messages. I DO NOT have a similar problem with my iPhone 12 Pro. It can send both iMessages and sms/mms on weaker signals using the T-Mobile (Mint) service. I am wondering if it is an OS issue. I have also found that the same SIM card in a Grapheneos Pixel 4a will NOT have problems with messages even though the signal is weak.