If you have suspended the cellular account, then the only way the device could be tracked is if it connects to Wi-Fi. For Find My iPhone to work, the device has to have Find My iPhone activated before it is lost/stolen, the device has to be powered on, and it has to have an active internet connection. If any of those is not in that state, then it cannot be tracked. Did the device have a passcode? Do you know what version of iOS? If it had a passcode, then the thief will not be able to get into the phone. And, if they try to restore it, if you have iOS 7 or above, then Activation Lock will prevent them from being able to activate the device, it will be useless to them.