How do you restore iPhone when it does not recognize the iPhone

Are you saying that when you attempt to restore your iPhone, by connecting to a Mac or PC running iTunes, that iTunes does not recognize your phone?

If so:

• "On your Mac, make sure that you’ve installed the latest software updates (including iTunes if you have MacOS Mojave or earlier). Or on your Windows PC, make sure that you’ve installed the latest version of iTunes." - Apple Support
• Do you see any error messages or just that iTunes does not recognize the phone?
• Be sure iTunes it not running when you connect your phone.
• Try another USB cable.