I assume you have the ZW just hooked to Fixed 1 Input and your track to Fixed 1 Output.
To operate on Fixed 2, you must apply power (red/black) to Fixed Input 2 and move the track wires to Fixed 2 Output.
I suggest the easiest is to run two jumpers from Red to Red and Black to Black on the Inputs. This way, the TIU has a source of power to run its internal electronics which it normally gets from the Fixed 1 Input.