Ultrasudio Express & Dell S2421N

I'm trying to get a new Dell S2421N monitor to work with my Ultrastudio Express via HDMI. It can detect the signal but shows no image.

The monitor supports RGB, YCbCr - but, it seems, not YUV. Is this likely to be a monitor incompatibility issue, please?

I'm using Desktop Video 12.7.1 on MacOS Monterey and an Intel based iMac.

Thanks very much

YCbCr is the digital version of YUV, and HDMI is digital. Unfortunately, not all HDMI devices are fully compatible.
I'd try a cheap HDMI splitter, it has helped others.