Firstly two things, have a lovely time with the grandchildren I am glad the handhelds work, secondly I am not trying to cut across all the advice that you have been given.
With the help of MTH I have been successful after 2 weeks of frustration, the advice I was given was to go back to basics
1. ensure all devices are the current REV
2. remove all devices from the layout taking power off the track
3. because it worked for me , power the TIU directly into the power socket on the side, I used a Lionel starter set power pack 18V DC ( I know MTH and others have said no need, but I am not convinced)
4. power up and make sure all the 3 lights on the WIU are on and steady, no flashing except the Wi-FI when it is connecting
5. put just one MTH engine on the track
6. power up and start the handheld app, you should be able to fined the engine and run it
Good luck
