If you were not using Find My, there is no way to track the stolen phone.
Carriers can track phones by IMEI but will only do so in unusual circumstances (e.g., if a child has been kidnapped, and the authorities have a court order). They won't do it for phones that are merely lost or stolen. Apple does not have any tracking mechanism except Find My, which you indicate that you didn't use.
You can ask your carrier to blacklist the stolen phone. If the carrier blacklists the phone, no carrier who honors the blacklist will provide cellular service to that phone again. This won't brick the phone as completely as Find My and Lost Mode would, but may still make the phone less valuable to the thieves.