We would suggest you to follow the mentioned steps to resolve the issue:
1) Ensure that network frequency of your device is 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 also try using different network operators.
If still issue persists then we would suggest you to contact your service provider.
For further assistance, register your concern in Samsung members application (Open Samsung members app >> click on “+“icon > send feedback >> error report / ask question).
Team Samsung Members