It is very unlikely that the Tamiya equipment is defective (possible, but very unlikley). Tamiya electronics are very durable.

It is advisable to verify that your radio system is working correctly before looking at the tank's electronics. I've told you how to verify the radio is sending and receiving signals (battery and servo trick).

Before you verify the radio, please check your tank's 7.2v battery charge by removing it, and recharging it fully (regardless of whether you think it is charged or not). A little known fact is that Tamiya DMDs have an integral low voltage cut off which protects the electronics from a brown out (low voltage). If there's low voltage situation, the DMD will internally cut off the rudder channel power to the DMD. The DMD will exhibit symptoms akin to what you describe yours to be doing (tank does nothing, and has two flashing orange lights on the DMD).