T-Mobile 5G Gateway doesn’t work anymore with Yealink SIP-T58W VolP phone

About a year ago I installed T-Mobile's 5G Gateway at my other property. It worked great! My daughter moved into the house and alog with her, her work equipment: computer and VolP phone. It worked great for a few months. Two weeks ago the VolP phone stopped working. The past 12 days I've been working with T-Mobile and the business phone folks, Temecula Valley Communications, to resolve the problem. Both blame the other. Today I swapped out the original 5G Gateway for a later model. Internet works fine, the phone does not. TVC did a hard reboot on the phone. That did not resolve the issue. The phone company says T-Mobile must have SIP on, no way to tell. HELP!