Quick update:
We arrived BVI on Monday Aug 15 -- using ATT International Day Pass for 11 days trip.
Service for the most part has been useable (LTE), with notable exceptions:

1. Western shore of Virgin Gorda - at best 2 bars of 3G (Flow). Not enough throughput to hotspot the laptop. We anchored in Trunk Bay (just north of The Baths National Park) and service was, as expected, zero. Next night we anchored in Pond Bay. Also zero service. Fine. It's vacay, and we chose to anchor in a secluded place.

2. Anegada. Same. Although we had telephone and even whatsapp kinda worked (kinda, not exactly a technical term) - our 48+ hours in Anegada were "turn it all off and enjoy being in Anegada. Nothing wrong with that.

3. The Dogs -- why did I try to make a call? Because there was an urgent matter. That had to wait.

4. Guana Island - White Bay - 4 bars of LTE, hotspot is perfect. There must be a tower at the resort.

5. JVD -- LTE, even on our overnight at Sandy Spit

6. Norman Island (not The Bight) -- not so good. Able to call, but limited data. No hotspot.

7. Peter Island Great Harbor, 3 bars LTE, good data.

Everywhere else near Tortola was strong enough LTE, and I paid $10/day for unlimited voice/text/data + up to 40GB hotspot per month, for the first line, then $5 per line, up to 10-days per line.

Overall, no complaints, and we didn't have to buy a subscription to anything beyond our ATT plan.
I hope this info is useful to others in the future.