The enclosed environment is a common reason for cell signal obstruction.
If you are in densely spaced buildings or an enclosed environment, you are not likely to receive a good signal.
The coverage of the network could also differ in operators.
If the signal issue only occurred in a certain zone, or it also happened to a friend whose SIM card was supported by the same operator, the signal weakness is possibly caused by the lack of network coverage. Please try to move to other areas with better signals.