U4323QE, problems with KVM

Just got one of these monitors. I can't get the KVM functionality working. 

I have a desktop (windows 11) with a video connection from HDMI on the computer to HDMI1on the monitor, and a USB connection from USB-A at the computer to USB-C (data only) on the monitor. The second computer is a 2015 MacBook Pro with the video connection from Thunderbolt 2 (mini displayport) through an adapter to HDMI to HDMI2 on the monitor, and USB-A at the macbook to USB-C (data only) on the monitor. 

My keyboard and mouse are both wireless with proprietary dongles that I've put in the two USB-A slots on the monitor. 

When using the input source, this setup works fine - I can use the wireless keyboard and mouse for the desktop, and the laptop's keyboard and trackpad, switch the display between the sources, do all the display tiling stuff.  When I try to set up the KVM functionality, I get as far as a the stage where it needs to move the mouse... and it doesn't. 

Any clues where I've gone wrong?