Please try these steps:
- From Settings > Connections > Mobile Networks > Network Operators > Search for network or search manual > Select the right network.
- From Network Mode > LTE Auto connect.
- Then Restart the phone.
- If the same issue exists, please open Settings > General management > Reset > Reset network settings > Enter the PIN of the screen > Reset network settings.
- Try another SIM card.
-Otherwise, please contact the network provider.
If the issue still exists, you may need to submit your device to one of our authorized service centers in order to better assist you using below link.