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Go to Settings/General/About and look at Modem Firmware. Is the entry blank? If it is you need to reinstall it by restoring the phone to factory settings, then restoring a backup. Back up the phone first, of course:
- How to back up your iPhone, iPad, and iPod touch - Apple Support
- Restore your iPhone, iPad, or iPod to factory settings - Apple Support
- Restore your iPhone, iPad, or iPod touch from a backup - Apple Support
If the modem firmware DOES have a value you may have a hardware problem. The first step is what I just posted, but after that you need to contact Apple support using the Get Support link at the top of this page.