JJC JF-U wireless trigger does not trigger flash at the right moment

I am trying to take a photo with my camera and trigger an off camera flash all wirelessly with the following setup:

A Nikon D5600 and a Metz B520-AF flash which I’m trying to use with a JJC wireless trigger kit (like this one http://www.jjc.cc/index.php/Product/product_info/id/575). On one of the receivers I attach my flash while the other one is connected to the camera with a trigger cable. I have to use manual mode on the flash because TTL mode is not supported by the trigger kit. When I press the button of the transmitter the flash fires and the camera takes a photo.
The problem is that the flash and camera don’t appear to be synchronized – no light from the flash seems to be present at the time of taking the photo and my photos turn out dark. The only way I manage to get it to work is when I have the transmitter attached on top of the camera and I press the button of the camera itself but this effectively defeats my purpose.

If someone could point out what I am doing wrong or explain how I could get it to work I would be eternally grateful!