My iPhone message reads: update required to access cellular data. What can I do to fix the problem?

As it’s an iPhone 7 first check this link→iPhone 7 Repair Program for "No Service" Issues - Apple Support

Next, try this→If you see a Cellular, Ultra Wideband, or Apple Pay Issue message after an iOS update or restart - Apple Support

And finally: 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 (which is not included in those instructions)
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.

I assume that you are using the latest update for the iPhone 7, which is IOS 15.7.1. If not, that would be the place to start.

Settings > General > Software Update

If you see No Service, Searching, or SOS on your iPhone or iPad - Apple Support

Sooner or later you are going to have to call your carrier. If it were me, that would be the first thing I would do. Chances are the "update" that is required will be provided by the carrier.

You must have found the solution since you marked your post as Best Answer. Nice work.

It would help other users who might be having the same issue if you could post your solution.

Thanks.

There is no content in your last message. Could you please re-post?