The Parts and Service History section of your iPhone's settings will show you if any parts of your iPhone have been replaced or repaired. If it says "Cellular Issue Detected", this means that iPhone has identified a problem with the cellular antenna in your iPhone.
If the Parts and Service History section says "Cellular Issue Detected", and you have already tried the troubleshooting steps above, then you will need to take your iPhone to an Apple Store or authorized repair center for further diagnosis and repair.

If your iPhone has a history of Camera/Display/Anetana replacement / Repair / Service from an un-authorised service provider then it is possible that Apple may refuse to service your iPhone if it has been previously serviced by an unauthorized service center.

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