Without seeing anything, it's impossible to determine what's wrong. If the receiver's getting proper power, and the transmitter's batteries are good, then the Tactic TR325 receiver should link with the Tactic TTX300 transmitter.

How is the receiver getting power? Directly from the car's battery pack? Through the ESC? From its own battery pack? What's the voltage being fed to the receiver?

You may need to send transmitter and receiver to the service center. It could be one or the other.