@Karsten12 

Did you check the Channel settings on the main NVR page?

So if the channel did not bind to a camera, it will show black screen with a tp-link logo on it. Click on the channel and there will be a "plus/add" icon to let you choose which camera to bind.

However if your "blackscreen" is a whole black, means there could be some issue on the camera.

  @Somnus 
Hi,

The cameras have already been added on the main page, it just shows a Black Image, there is no "+" symbol. Not sure what the problem could be. I have already tried reinstalling the cameras but that didn't fix it.