Sudden inability to roam on T-Mobiile using a Wind Mobile Samsung Galaxy S5

Problem: As title.

What doesn't work:

• Going to Settings / More networks / Mobile networks / Network operators and
• Re-scanning
• Trying to connect manually to T-Mobile
• Setting to manual selection, then back to automatic selection
• Going to the Wind SIM toolkit and trying anything there
• Rebooting
• Shutting down for some arbitrary interval (i.e. 30 seconds) then restarting
• Removing and replacing the SIM and restarting
• Having Wind turn USA roaming off and on again

What may work:  Changing "Settings / More networks / Mobile networks / Network mode" from "WCDMA/GSM (auto connect)" to "GSM only", re-scanning available networks, and manually connecting to T-Mobile.

No idea why this would work, but it did. 

It might even stay connected if you then return "Network mode" to "WCDMA/GSM (auto connect).  But I don't know if the original connection problem will re-occur the next time with this setting, or not.