I gave Mint a try a couple months ago on a spare phone I had.
Same experience u had, but in the Twin Cites, Minneapolis/St. Paul area.
Data speeds were ok, in the suburbs, but near useless the closer in to the cities, downtown area, or in large shopping malls, like The Mall of America.
Sure, Speedtests, like from the Speedtest.net app, showed good speeds, but I have known for long time that carriers almost always have those servers unblocked or un-throttled so that the speedtests always show fast speeds etc.
My RW phone, also on Tmobile, had full and faster LTE speeds in the same locations with no issue.
Mint also keeps trying to charge my credit card, despite my canceling the line.
The call in support cant help as they show the account closed…I have gotten ZERO responses to emails. I ended up just reporting it as fraud and got a new card number.
The same thing happend when i tried Unreal mobile.