Verizon mainly w/ Unl. Data, AT&T sometimes, HTC One M8 4Me?

[AC Question]

Due to my deep woods main residence, I have Verizon as they are the only reliable signal throughout the house. Also, only have 3.0 dsl available, so I use the Verizon LTE to supplement my bandwidth via FoxFi hotspot (grandfathered unlimited data). I now spend a decent amount of time in the US Virgin Islands, where AT&T is the right answer (they consider that domestic, unlike Verizon). So I would like to keep Verizon and my hotspot at home, then pop in an AT&T sim in the islands. I think this will work, looking at the specifications of the M8 (AT&T uses band 4 in the USVI). Is this correct? Is Unlocked good enough or do I need a Developer? TIA