Getting a cellular error message on my iPhone. What does it mean?

Cellular Update Failed —> If you see an Update Failed or Apple Pay Not Available alert after iOS update or restart

If you see No Service or Searching on your iPhone or iPad

1. Check your coverage area. Make sure that you're in an area with cellular network coverage. ... 
2. Restart your iPhone or iPad. Restart your device. ... 
3. Check for a Carrier Settings update. ... 
4. Take out the SIM card. ... 
5. Reset your Network Settings. ... 
6. Update your iPhone or iPad. ... 
7. Contact your carrier. ... 
8. Get more help.

If you see No Service or Searching on your iPhone or iPad ...

Remove your SIM card from the SIM card tray and then put the SIM card back. ... Make sure that the SIM tray closes completely and isn't loose. If you use a SIM tray from a different model iPhone, iPad, or from another phone manufacturer, it might not fit properly.

If you see 'invalid SIM' or 'no SIM' on your iPhone or iPad ...

Learn what to do if you see a SIM not supported alert.

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Apple Support Community Forum

This is Apple Support Community forum, primarily for a user to user interaction for information & idea sharing between users, if need be, extend support on technical know-how in handling Apple products from a more experienced fellow user, This is not the right forum to discuss with Apple Inc.