With no TIU in circuit and only one loco on a powered track. Put voltage to above 10. Press whistle button once followed by 5 bell button pushes at half-second intervals. Timing is critical. If you succeed, two toots. If not, try again. I'd try the Z4000.
I would not try factory resetting at this point.
How do you power your TIU? Through AUX port?
ANSWER: Power is supplied from the WiFi unit via the cable. Plugs into the top of the TIU.
I agree there's some obvious omission. Still don't know why son's TIU wouldn't work. It should have, if set to same ID address as yours.
Have you in your remote turned on the DCS signal????????
ANSWER: I’m not clear on this. The remote appears to be linked to the TIU since it can read the firmware version. I’m assuming this would be from the System category but, I’m not seeing anything that indicates turning on a DCS signal.
Edit to add I’ve scrolled to DCS SETUP; and clicked on All Outputs On. Still get no engine found.