Kindly follow the mentioned steps to resolve the issue in your device.
1) Ensure that network frequency of your device should be selected on Auto-mode.
Menu>Settings> Connections>More Networks> Mobile Networks> Network Mode> Select Auto mode (GSM/WCDMA/LTE).
2) Search for the Network Manually by the steps as mentioned:
Menu> Settings> Connections>More Networks> Mobile Networks> Network Operators> Select Manual Search.
3) You may check by exchanging the SIM in SIM slots and different SIM card in your mobile and check if the issue is resolved or not.
If still issue persists then we would suggest you to contact your service provider.
Team Samsung Members