iPhone No Service with warning icon
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.