How to track a stolen iPhone 15 pro when Find My iPhone is off?

Offline does not mean that Find My is off or was not turned on.

Offline means that Find My was turned on at the phone.....and.... that one or more of the following are true:

• The battery on your phone has run down
• The phone was powered off
• The phone does not have an Internet connection where it is currently located

Or.....stated another way.......your phone will appear on the Find My map......IF......all of the these things are true:

• The battery has ample power
• The phone is on
• The phone has an Internet connection

All that you can do is keep trying a few times each day to see if the status of the phone has changed. You might keep in mind though that iPhone thieves are very fond of wrapping a stolen phone in aluminum foil, so the phone cannot send or receive a signal.

I'm sorry, but realistically the chances of ever recovering a stolen phone are just a tiny fraction above zero.

Find My is the only way that a user can track the location of his phone. Your carrier can provide some location information, but they will normally only do this in cases of a missing person. Even then, the carrier will require a court order to proceed.

Good luck