To resolve network issue please follow below given steps:-
1. Check network operators and select automatically (Settings> Connections> Mobile Network>Network operator> select automatically > Tap on OK > Registered on network).
2. Reset the device: settings >General Management > Tap on Reset. Note: Before reset make sure to have back up of the data via Smart Switch Mobile App (Install Smart Switch App > Open > Click on Memory Card Icon >Back Up option > Choose the data for Back up > Back Up).
If the issue still persist, kindly register your concern in Samsung Members app (Samsung Members app > Get help > Feedback > Error report/ask question). Note: Please send device log files within 15 minutes of issue occurrence along with the exact issue details.