Dear Samsung Member,
Greetings from Samsung Customer Support!
Follow the bellow steps to resolve network issue:
1. Check device network in open area.
2. Check device network mode settings:
Settings > Connections > Mobile networks > network mode SIM1 or SIM2 > network mode.
3. 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.
4. Kindly do check same SIM card in another phone or another SIM card in same phone to verify the issue.
If issue still persist, kindly register your concern in Samsung members app (Samsung Member > Get help > send feedback > Ask question/Error report)
Thank you for writing to Samsung.
Samsung Customer Support