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I got it fixed, after many calls to both Verizon and TMobile tech support, and visits to stores. It turned out to be very simple, and it came from a CUSTOMER in the TMobile store in Santa Cruz:
1. Go to Settings, and then Mobile Networks
2. Got to Access Point Names
3. Select your APN that might look like T-Mobile US LTE, and make sure these are all like this:
Name = T-Mobile US LTE
APN = fast.t-mobile.com
APN type: default,supl,mms <-- this was the secret that finally made MMS start working!