Can't open apps on iPhone when connected to cellular

Is the option to use cellular data activated in Settings/Cellular?

If you scroll down that page on this phone, you should see apps listed, that are using cellular data, and the use should be enabled right next to the name of the app.

Is the SIM card recognized by the iPhone?

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

If set to 5G, try to set it back to 4G

Uninstalling iOS 16.2 is not supported by Apple.

But, you can try to set it back to factory settings, using your computer.

That way a completely new iOS software will be downloaded from Apple and installed, including all needed updates, for example the carrier updates for the use of the cellular network.

Restore your iPhone, iPad or iPod touch to factory settings

Make sure to back up your phone before you start.

Set it up as a new device first, without using the latest backup and try to open an app, for example Safari, using the cellular network.

If it works, use the backup to restore your data and settings. Then try again to open apps.

Restore your iPhone, iPad, or iPod touch from a backup - Apple Support

You're welcome 🙂

Glad that he has a working phone again.