Please perform the below mentioned steps to resolve the issue:
1. Check network operator settings Settings > Connections > Mobile networks > Network operators > Select automatically > OK > Registered on network.
Note: We recommend to set device network operators as LTE/3G/2G (auto connect) mode.
For further assistance, register your concern in Samsung Members Application (Open Samsung Members Application > Get help > Send Feedback > Error report/Ask questions).
I have also done same mistake of buying M31. Airtel 4G is working with very good signal strength in iPhone 6, at the same place. M31 is working only E (2G data). Tried changing all settings, but failed to get 4G / LTE, even for a minute.
