This is the short version for now.
T-Mobile support could not get data service to work with an unlocked Samsung Galaxy S7 Edge SM-G935A ATT Phone.
Problem is there are too many mobile networks vying to connect you to data service.
Here is the solution.
Settings -> Connections -> Mobile Networks -> Mobile Data -> Make sure it's on! <- Back
-> Access Point Names ->Ellipses -> Reset to Default
Select and delete EVERY acess point except for default selection by selecting APN -> Ellipses -> Delete APN
Delete every one except default selected APN. Then make sure you have the following settings.
Access Point Name (APN): fast.t-mobile.com
MMSC Server: https://mms.msg.eng.t-mobile.com/mms/wapenc
APN Type: default,supl,mms
APN Protocol: IPv6
APN Roaming Protocol: IPv4
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