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.
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