Have you set the mixer utility inputs to Control Tone Line and the mixer channel selectors to PC?
This is what it should look like (although this is for my DJM-900NXS2, and it's currently disconnected, but the inputs should be the same):
You may simply need to quit rekordbox, check the settings in the utility (above) and then re-launch rekordbox. The other thing you may need to do is toggle the input from Control Tone Line to another non-control tone setting, then toggle it back. You should hear a slight "click" from the mixer, then launch rekordbox and try again.