How can I track stolen iPhone?

It's almost impossible to track a phone which is off and not connected to a network. Even more difficult since it's been 3 months. Do what is suggested in this link, but the chances of that phone ever being recovered are slim to none --> If your iPhone or iPad was stolen - Apple Support

Even if you had that iPhone registered with FindMy, there is likely very little you can do at this point, especially if it is turned off. I've copied the link to Apple Support for FindMy below. You may want to contact your insurance company.

Use Find My to locate your lost Apple device or AirTag - Apple Support (ME)