I do not find any details on service commitment... it is buy for $200, pay for 1 mo svc, then cancel... effectively a $240 iPhone SE for use with FP LTE SIM?
I think these are locked but since you want to use it with a FP sim you should be fine to cancel after 1 month.
I would think about the virgin 1 year free deal though - $279 for the iphone SE and $1 for a year of service. You are supposed to be able to unlock right away just by calling in. Of course if sprint is useless for you then no point in that. Also you'd get the sprint model which has slightly different lte bands too but would be fine for ATT as far as I can tell.
It is a prepaid iPhone that is being sold by a retailer. That means you are pretty much on your own if you do not like whatever limitations it has.
If you go ahead you probably should test everything you want it to do within the return period.
That way the worst case scenario is the restocking fee.