ok, so when you say you connected power, you did positive to the Red and Common to Black correct? I'm not sure what the red light means on the WTIU, but I'm going to assume there is a short somewhere. I would start simple, find a small loop of track or clear your whole track except 1 DCS locomotive.
Can you post some more about how you have the WTIU wired in? Should be wires from the Transformer into the In, the from the Out side to your distribution block / track.
Also, Fixed just means it will pass through whatever voltage is passed in, so if you crank your handles up to 19 volts, that will be passed to the track, if you only go to 10, only 10 will be passed to the track. Variable channels allow you to adjust them as if they are handles. i.e. you pass in 19 volts but use the remote or app to change the voltage to the track. Keep in mind to use the variable channels like that you will need the premium side of the app. Also, that will allow you to change the variable channels to fixed if you'd like.
The WTIU will do nothing for your Lionel locomotives, you need the Base3 for Lionel, the WTIU will only run DCS locomotives (unless you link the Base3 through the WTIU with a cable coming in the future, but given your issues keep it simple) leave your Lionel engines in run and use them as normal using the Base3, DCS and Legacy can run on the same track, the signals don't interfere with each other.
For the DCS locomotive you put it on the track, add power, get the purple light on the WTIU for that channel, then in the app you "add" the train in.
Also, I added the WTIU manual, it isn't great but has some basics about adding and running engines.
