Problem using rmx 1000 app

Hello, I am writing because I'm a little desperate with a problem I have with rmx 1000 app for iPad air 2. I have bought the things I need to use it with my djm 850. I bought a Numark soundcard iOS class compliant, which I know that works perfectly with the app, the lighting usb cable for the iPad and the rest of cables I need. I tried to use it with the "send and return" which is the thing I am interested in the most. I connected it perfectly and the app even detect the input and output signal and even perceive the bpm I'm playing (as you can see in the video attached), but when I active send and return, it does not sound anymore. I tried it with my djm and my two xdj 700, and also with traktor. I really tried all possibilities... even connecting the input of the sound card with the master of the mixer, and the output of the sound card with my speakers, and nothing happens (the app also seems to detect the signal in this case).

If you could help me it would be great, maybe I am making something wrong, as I saw some video in which it works as I am using it... Otherwise I would have to restore the app and things I bought.

Here you have the link with the video I recorded:

https://wetransfer.com/downloads/d4f9c645a884a32576b8924b38ad260c20180109204952/c031bc26f43450bb64369b14c0b3fae520180109204952/24b83a

Thank you so much, I wait your answer.