Most trips start at Philip Goldson International Airport just outside Belize City. By the time you taxi to the gate, your phone should already show a signal if you turned the eSIM on during the flight. The eSIM connects to DigiCell, the network run by Belize Telemedia, and it has the widest reach in the country.
Belize City itself has steady 4G in the downtown core, around the Swing Bridge, and near the water taxi terminals where most people catch boats to the islands. This is the easiest place in the whole country to load a map, book a ferry, or message your hotel. If you only need data for a quick overnight before heading out, the 1 GB for 1 day plan at $10.78 covers it.