How to turn on a stolen, offline iPhone 15 Pro?

You can’t turn it on remotely.

If Find My Network was enabled, and another Apple device comes within very close proximity, that device might report a location. But even if you saw a location, what would you do with it? Confront criminals in person and maybe lose a lot more than your phone?

As others have said, you cannot turn it on remotely.

Read this support article about what to do if a iPhone or iPad is lost or stolen -->

If your iPhone or iPad was stolen - Apple Support

Mentioned in another post:

The chances of locating a device depend upon which features you selected in: How to turn on Find My on your iPhone, iPad, or Mac - Apple Support

"To see your device even when it's offline, turn on Find My network. The Find My network is an encrypted, anonymous network of millions of Apple devices that can help you locate your device."

And from: Add your iPhone to Find My - Apple Support

"On a supported iPhone*, turning on the Find My network allows you to locate your iPhone for up to 24 hours after it’s turned off, or up to 5 hours when it’s in power reserve mode."

If Find My is offline or not working, read this Apple Support article to get help; e.g., when you don't see your device listed, or you see an unexpected device or an alert that says your device is offline. --> If Find My is offline or not working - Apple Support

You can't turn it on remotely.

If it was stolen you will never see it again, just buy another.