Let's assume that everything you suspect is correct: someone has got into the phone, has reset it and is using it with your SIM (because if your data use has increased it must be your sim being used).
If it's been reset and is no longer connected to your Google account I doubt you can track it: even if you had a tracking tool installed and enabled it would be removed by the reset. But your carrier, and the police, can track it if it is using your SIM. Actually they could identify it from the IMEI too.
But if you think it is being used that implies that you have not reported it as stolen. If you had then either the police would track it or the carrier would kill the sim and blacklist the phone (rendering it unusable on any network). Which would depend on the circumstances: I know a case where they left a stolen phone live in order to trace the thief, but it involved a mugging so may be treated differently from "it's gone missing and I think someone took it".
But frankly unless you think it's still going to turn up behind the sofa the only person you help by not reporting it is the presumed thief. And if you have reported it and your data usage is still going up then it is probably something else, since both sim and phone should be disabled.
(Of course another possibility is that someone has not got into your phone but has decided to use the sim for free data until you do something about it. Just considering all the possibilities).