Dear Samsung Member,
Greetings from Samsung Customer Support!
Please perform the below mentioned steps to resolve the issue:
1. Check network operator settings
Settings > Connections > Mobile networks > Network operators > Select automatically > OK > Registered on network.
Note: We recommend to set device network operators as LTE/3G/2G (auto connect) mode.
2. Check SIM cards in other smartphone.
For further assistance, register your concern in Samsung Members Application (Open Samsung Members Application > Get help > Send Feedback > Error report/Ask questions).
Thank you for writing to Samsung.
Samsung Customer Support