IPhone 7 is disabled because of a unknown passcode and the carrier account has been closed. How do I reset the phone?

because of unknown passcode

(< iOS 15.2) See this Apple article: If you forgot your iPhone passcode

If you forgot your iPhone passcode - Apple Support

(iOS 15.2 and newer) 

How to remove your iPhone, iPad, or iPod touch passcode without a computer

How to remove your iPhone, iPad, or iPod touch passcode without a computer - Apple Support

and no carrier account

Look for / visit a carrier, take a subscription, postpaid or prepaid card.

Checks whether the SIM card is suitable for your iPhone.

See this Apple article: If you see No Service or Searching on your iPhone or iPad

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

See -> iPhone 7 Repair Program for "No Service" Issues

iPhone 7 Repair Program for "No Service" Issues - Apple Support