DDJ-400 Headphones not working on external sound card

Hi guys, I'm having a problem with my DDJ-400.. I use to do some gigs with it, however I realized that it has a much lower output than other entry level controllers, so I tought of using my external soundcard (Focusrite Scarlett Solo 2nd Gen) as a 2nd gain stage to output the sound coming from my controler to the P.A system. So far so god, I pluged the Scarlett Solo, opened RB and changed the audio output configurations on RB settings to my external soundcard instead of the DDJ-400. The output sound immediately become more noticeable and with the added bonus of having an output volume knob. I was so happy believing I had figured out a nice workaround until the moment I plugged the headphones to the DDJ-400 and realized that I was unable to CUE the tracks because there was no sound coming out. I've turned all the knobs and buttons of the CUE/Headphones Mixing and Level with no sucsess. I also tried to plug the headphone jack to the Scarllet Solo instead but I was only able to hear the master output from there. Is there any way to fix this, or am I missing something? Thanks a lot!