City and pier

Ha Long city is not remote. Bai Chay, the cruise piers, and the road in from the Hanoi expressway are covered the way other provincial cities are covered. You can land the Grab pin, check the boat ticket, and message the operator from the harbor. Do those things at the pier. The bay itself is limestone and water, which is a poor radio environment no matter whose logo is on the SIM.

On the water

A day cruise and an overnight junk both leave the dense grid behind. You may see LTE near a floating village and then nothing in a channel between tall karsts. The phone will hunt, warm up, and drain. Airplane mode, or simply accepting that you are offline, is kinder than watching it search for three hours. Nothing about that search deletes the eSIM or resets your plan. When the boat returns to the pier, the same line attaches again and uses whatever remains of the day’s 5GB.

Cat Ba is a separate hop, by ferry or by the newer road links, and it behaves like a small island town: the center is fine, the coast road is mixed. Viettel remains the more plausible network if you are also going into the hills behind the beaches. Bring the offline map anyway.

Data math here is simple. The harbor hour is tiny. The mistake is starting a cloud backup on the boat during the ten minutes of signal you do get, then watching it fail and retry. Turn photo backup off before you board. 5GB will be waiting in the city afterward, unless you already spent it on the bus from Hanoi with a laptop on hotspot. The reset remains 23:59 Vietnam time.