Video Capture from HDMI out on ATEM Mini

I have successfully plugged a TV into the HDMI port on the Mini and it makes a perfect monitor and works at it should with two cameras. However I have just purchased a small Video Capture Card HDMI to USB but although OBS shows the capture card as a source I cannot get an image. The capture card works fine with a camera connected directly (not through the Mini) into OBS. Am I trying to do something that is impossible? Is it the wrong sort of capture card?

Any advice would be much appreciated.
Thank you.
Andrew

Have you tried to change the format for the Atem? Perhaps the converter does not support the current setting