Posting here as it seems to be a common problem that you lose MMS text message function when changing over to T-Mobile from Verizon with Samsung S7. I search through the forum (though maybe its here somewhere), many times on the phone with T-Mobile tech support / Samsung tech support (both mostly useless) trying many different APN settings/factory reset 3rd party messenger applications but no luck.
Luckily the store in which a purchased my new plan was very helpful. Ultimately these APN settings were the ones that had to be changed to function correctly sending/receiving MMS messages:
APN Settings required to change from the default:
APN roaming protocol:
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