I'd like to try being some help since I did some research on LTE/4G frequencies before I last entered the US (I'm from Europe). The ATT Unite Mifi can handle 3G speed in the areas we're talking about - of course if it's not SIM locked. Talking of highspeed (LTE/4G) it's far more complicated but as far as I know right now the BVI does not have real LTE/4G. What they offer (afaik) is really optimized 3G. They should have some real LTE in the first half of 2016.

Then, you would need not only a unlocked MIFI - you need to have one with the right 4G bands. The Unite Mifi uses 4G bands: 2,4,5,17 - for example in Antigua&Barbuda Digicel uses Band 17 while Lime is on Band 4.

To wrap up my first lines: For the trip to US I ended up buying an used unlocked AT&T Unite Mifi for 25$ since my european Mifi lacked the nec. bands... Complicated world.