Is there a real easy solution to restore the iPhone modem firmware?

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:

1. How to back up your iPhone, iPad, and iPod touch - Apple Support
2. Restore your iPhone, iPad, or iPod to factory settings - Apple Support - follow the instructions to restore using your computer, do not using Settings/General/Reset
3. 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.

Mohamedgg Said:

"Is there a real easy solution to restore the iPhone modem firmware?: iPhone 13 , my modem firmware went missing, and now I'm having cellular issues always searching for network but never gets and IMEI doesn't show up when dialing *#06#. Is there a real easy solution to restore the modem firmware?"

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Perform an Update:

See if performing an update installs any of this went-missing firmware. Have a Computer Handy? If so, try using a computer to update. Go here: Update your iPhone Using a Computer - User Tip. If any updates are availabe, they will install, sure enough.