Solution No.1: Toggle Airplane mode
To do this, go to Settings, turn on the Airplane mode, and turn it off once more.
Solution No.2: Reboot your iPhone
If your iPhone is not rebooted recently, that may cause some issues. You can try by holding Wake / Sleep buton + power button to restart. You can see Apple logo after restarting started and then your can see the Home screen.
Solution No.3: Reset Network Settings
This is the simple step as well. Navigate to Settings, click on General > Reset > Reset Network Settings.
Solution No.4: Set “Data only”
This step helped many people. Go to Settings, tap on Cellular, pick on Enable LTE, and tap on Data Only.
Solution No.5: Check for carrier settings update
To do this, ensure you are connected to the Internet. Tap on Settings, General, and after that About. This step does not require an excessive amount of time or exertion, but rather it will be greatly valuable.