I have this same issue now that I've just added the LCS WiFi module to my Legacy & Original Cab1 TMCC setup with TPC300 (original TPC from IC Controls) controlling track power and plugged into an LCS SER2 module.
My TPC300 is set as TR id #1. I have no problem using my Legacy or CAB1 remote and pressing "TR" followed by "1" and then turning the red knob clockwise to ramp up power to the track connected to the TPC300. But I can't figure out how to do this with the iCAB application on my iPhone. Once I use the Legacy or CAB1 remote to address the TPC and power up the track I have no problems using the iCAB app to control my TMCC trains. So overall I can tell all the components are linked together and are functioning. I just can't access the TPC via the iCAB app.
Is this "working as designed" and it just can't be done? Or is there some other trick to getting it to work? This unanswered thread was all I could come up with when I searched.