Your MegaEsim plan runs on Glo, also called Globacom. Glo was the first operator in Nigeria to roll out 4G LTE across the country, and it now covers all 36 states. So you are riding on a network that was built for wide reach, not a small regional carrier.
One reason Glo travels well is the radio band it uses. A good part of its 4G runs on the 700MHz band. That low band carries the signal farther from each tower and pushes through walls better. In plain terms, you tend to get a usable signal indoors and on the edge of towns, not just standing in the open.