My phone number

Any restrictions set up in ScreenTime? If yes, disable them.

Have you been able to use iMessage and FaceTime at some point in the past, using your mobile Network?

Does your carrier support iMessage?

Are your carrier setting up to date?

Manually update your carrier settings on your iPhone or iPad - Apple Support

To rule out a software issue, set your phone back to factory settings, using your computer.

Then set it up manually, without using the backup. See if you can select your phone number in the settings for iMessage after that.

Restore your iPhone, iPad, or iPod to factory settings - Apple Support