I was having the same problem using a Futaba 9CAP... also channel 5. I hooked it up on channels 1 thru 4, the light on RCATS-100 lights up when I move the sticks to one of the sides. I finally got it to work by decreasing the end-point adjustment from 100 % to 90%. Maybe it needs to detect one side that is not at full deflection. I'm surpised this hasn't come up before.

Opps...I just read the instructions again.

"Hint: may need to adjust ATV on your transmitter in order to set trip point. LED under label will light when relay is closed"

duh...