Turn off Airplane Mode. Make sure that the Airplane mode is turned off. Simply go to Settings->Airplane Mode on your iPhone. If necessary, toggle the switch to turn the function off.

Set the network settings to default. Resetting your network settings will also reset network information, such as your Wi-Fi network and passwords, cellular settings, and VPN and APN settings. As a result, make a copy of this information before you start. Go to Settings-> General-> Reset-> Reset Network Settings to reset your network settings.