Is the LTE sim on AT&T and the Global sim on T-Mobile?
FP LTE SIM = AT&T LTE and 3G
Global SIM = T-Mobile 3G
Until recently, the Global SIMs were both AT&T and T-Mobile. Nearly all reports I've seen indicate that AT&T isn't working anymore.
For what it's worth, I think this is probably a change in 3UK's roaming agreements, and not something FreedomPop decided to do.
Here's a 3UK page showing US roaming coverage:
compared to MintSIM (T-Mobile MVNO) US coverage map (which includes LTE)
The 2 maps appear similar.
Yes three used to actually list ATT and Tmobile by name but the map and user reports seem to indicate t-mobile only now.