Simply go to Settings, turn of Airplane mode, and after ten to fifteen seconds, turn it off again.
The other thing that may fix this issue is checking for the available carrier-settings update. First, connect to the available Wi-Fi network. Go to Settings, tap on General, and tap on About. If there are any updates, install them.
Press and hold the sleep/wake button on your device, and when you see the slider, drag it in order to turn off your device. Wait for a couple of seconds, and repeat the action to turn the device on again.
To do this, go to Settings, then General, tap on Reset and choose Reset Network Settings. All the settings you have will be restarted, including VPN and APN settings, saved Wi-Fi passwords, and Bluetooth records.
Before you start, make sure to back up your data on iCloud. Click on Settings > General > Reset > Erase All Content and Settings.