I can answer that one, we were just in the USVI a couple weeks ago. I had very good AT&T LTE service
all along the south and west sides of St Thomas.
St John was tougher, the towers were around Cruz Bay and that extended south somewhat. The mountains north of the Bay blocked signal all along the north coast. In the bays at Maho and Waterlemon, my phone
kept linking up to the Lime network over on Tortola in the BVI
. Tried to manually block that network but it was a company phone
and kept defaulting. Inadvertently ran up some data roaming, not sure what the cost was.
Also had signal at Skinny Legs in Coral
Bay St John, but we took a taxi so not sure about out on the water
No signal at all at Lameshur, but a lovely view at night of St Croix 30 miles distant on the horizon. Time to put down the screens...