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
Menu> Settings> Connections>More Networks> Mobile Networks> Network Operators> Select Manual Search.
3) Perform the reset network setting (Go to the settings>>general management>>reset>>reset network settings).
If still issue persists then we would suggest you to contact your service provider.
For further assistance, register your concern in Samsung members app (click on “+“icon>send feedback>Error report/ Ask Questions).
Team Samsung Members