Enclosed environments are a common reason for cell signal obstruction. If you are in a densely
spaced building or an enclosed environment, you may not have good signal.
The coverage of the network could also differ among operators. If the signal issue only occurs in a certain zone, or it also happens to people nearby whose SIM cards are supported by the same operator, the weak signal is possibly caused by the lack of network coverage. Please try to move to other areas with better coverage.