How to factory reset iphone12 when is screen lock and with out a computer

If your iPhone is running iOS 15.2 or higher......it probably is, unless you have not updated the phone in a long time.....you don't need a computer to erase / reset the phone.

If you see an iPhone or iPad unavailable message or Security Lockout screen - Apple Support

Once the phone has been reset, you can use a current backup to restore the phone.