How to reinstall iOS?

These instructions will restore iOS to factory settings→If your device has iOS 15.2 or later installed, you can reset it without a computer; check out this link→How to reset your iPhone, iPad, or iPod touch when you forgot your passcode - Apple Support. Otherwise you will need a computer:

See this Apple support link→If you forgot the passcode on your iPhone, or your iPhone is disabled - Apple Support

Or watch this companion video ⬇️

After restoring the phone using one of the methods above you can restore your most recent backup.

Reinstalling iOS will not help If you see a Cellular, Ultra Wideband, or Apple Pay Issue message after an iOS update or restart

Do you see a Cellular, Ultra Wideband, or Apple Pay issue message on your phone?

If not, you need to erase the phone. Make sure you have a backup before you do this.

Settings > General > Transfer or Reset iPhone > Erase All Content and Settings