I reflashed my L5, with updated chatty version 0.7.0rc5, mmsdtng version 1.13.0, system: Linux pureos 6.1.0-1-librem5 and sending and receiving SMS/MMS began working with T-Mobile.
However, about a week ago, my ability to send SMS abruptly stopped, although I could still receive SMS.
When I took the SIM card out of the L5 and reinserted it back into the iPhone, interestingly the SMS generated on the L5 was then subsequently received by the other party.