Tracking a stolen offline iPhone 13

Start here:

Locate a device in Find My on iPad - Apple Support

Locate a device in Find Devices on iCloud.com - Apple Support

Here is Apple’s advice for lost or stolen devices:

If your iPhone, iPad or iPod touch is lost or stolen – Apple Support

Use the Find My app to locate a lost or stolen device – Apple Support

If the Find My iPhone feature was enabled prior to it being “misplaced”, then if active, you would stand a good chance of locating it. 

However, if the feature was not activated prior to being misplaced - or the iPhone has been inactive for more than 24 hours, the last known location will not be available - and the opportunity to use technical means to recover the iPhone may have been lost.

If you need to find the serial number or IMEI for your iPhone, sign-in to your Apple Account using a web browser:

https://account.apple.com

Having signed-in to iCloud, access the Devices tab - where you will find details of all devices associated with your account.

The only way to track or locate it is using find my. If you had the find my network enabled, it can report its location when powered off, so long as it’s not in a faraday bag or something that blocks the signal, just as AirTags do. That’s provided there is at least some battery power and someone with an iphone with the find my network enabled comes within range for a few minutes.

You can’t track a iPhone using the serial number.

if it doesn’t have a network connection or is turned off, how do you imagine the phone would report its location.