AT&T Calypso

I have an unlocked AT&T Calypso U318AA. With AT&T service, the hotspot worked fine up until 5 days ago, but since switching to T-Mobile, it doesn't. It gets this error every time you turn on the hotspot: "Looks like there is a temporary network issue affecting mobile hotspot right now, please try it again at a later time." I looked up the APN settings for T-Mobile BYOD and saw only one difference. I created another APN with the exact same settings plus added the suggested MMSC value, but it made no difference. I've rebooted multiple times, tweaked all the hotspot settings one at a time, etc, but no change. I see this same error is occuring on BLU G91 Pro phones (via at 5 different Amazon reviews) since a forced software update in July 2...
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