Nevermind i figure it out myself after some hours of session in the studio. I was able to replicate the "issue" ( not really an issue now ). The thing is when you set the CDJ available on the Rekordbox, even if its the left player (supposedly deck 1) it enters to the "Output channels" in "Preferences" of RekordBox as Channel 2 (deck 2). So this is why no signal is visible on the mixer since on channel 2 is playing the previous DJ track, it makes sense, of course it can not just override the playing signal :) from USB or Ethernet Link. So the workaround is to check "Output channels" in "Preferences" of RekordBox and switch the channels output, for example where says "CDJ/XDJ 1" you choose "CDJ/XDJ 2" and that will do the trick, you will finally see the signal coming thru the mixer channel #1.
No need for more help here :) Thanks!