DDJ 400 - Focusrite 4i2 can't use headphones

Hello, 

I selected in options (recordbox) - audio - "focusrite usb audio".

Because if I select DDJ 400 wasapi, there's latency.

So the audio through the monitors is OK, but I can't find the way to use correctly my headphones. They are connected to the focusrite, and the headphones knobs from the DDJ 400 doesn't respond at all. 

in output channels there's a slot to select the headphones output, but have not worked any of them.