Try what Richie C suggests. Specifically, power your test track/loop from the Fixed 2 output. As I understand it, you have a Z-500 brick source from a starter set.
Since your TIU itself is powered by the Z-500 brick thru the Fixed 1 input, you'll still need this connection. Hence, run 2 jumpers from the Red and Black Fixed 1 inputs over to the Fixed 2 inputs. And then power your track from the Fixed 2 output. It's a long-shot since presumably you purchased a working/new TIU...but I suppose the communications circuitry for the Fixed 1 output might be compromised while the Fixed 2 output might be 100% good to go. There is separate DCS communications circuitry for Fixed 1 and Fixed 2.
Separately, as Joe suggests, it's always helpful to know the exact SKU or item number of your engine(s). I believe you have so-called PS3 (Protosound 3) engines since you said you could "reset" the engine using the starter-set DCS Remote Commander. That you can control your engines using the DCS Remote Commander is probably the most useful data point. In any case, while this may seem like a game of 20-questions, this is par for the course!