The compressor is always on for the external signal. The external input is a stereo input btw, if you plug a 1/4th inch mono jack you’ll only get the left channel…

Also, you need to give a really high gain signal to the external input in order to reach a usable range with the compressor IME. Not impossible at all, but you might need some external gain device (preamp, mixer etc) in order to output enough gain into the input.