iPhone No Service with warning icon

I have an iPhone 7, after updating iOS 12, it loses cellular signal, displays No Service with a warning icon. And an alert said:

Cellular update failed
Your iPhone cannot make and recieve calls or access cellular data until it has been updated

So what caused this problem? I searched on Google and only get 1 post about this but no good answer of how to solve it.

Solution 1:

Check for a cellular settings update:

You can manually check for and install a carrier settings update with these steps:

Make sure your device is connected to a Wi-Fi or cellular network.

Tap Settings > General > About. If an update is available, you'll see an option to update your carrier settings.

Try updating your device:

Plug your device into power and connect to the Internet with Wi-Fi.

Tap Settings > General > Software Update.

Tap Download and Install. If a message asks to temporarily remove apps because iOS needs more space for the update, tap Continue or Cancel. Later, iOS will reinstall apps that it removed. If you tap Cancel, learn what to do next.

To update now, tap Install. Or you can tap Later and choose Install Tonight or Remind Me Later. If you tap Install Tonight, just plug your iOS device into power before you go to sleep. Your device will update automatically overnight.