U2422HE, No USB-C signal from your device, #2

I have two laptops. My Dell Inspiron is less than a year old, and it's my personal machine. My HP EliteBook is about a year old, and it's my work machine.

I've been using a secondhand Dell U2721DE plus a hub. It's working fine, but I want more screen and fewer chunky cables, so I bought a new Dell U2422HE. The fellow on the Dell chat said that I'd be able to daisy chain the older monitor.

Unfortunately, I can't even get the new U2422HE to see either of my laptops via USB-C. I'm using the cable that came with the U2422HE. I've tried having the monitor set to AUTO and to USB-C 90W. I've switched the cable between my laptops and the same thing happens.

Weirdly, I have the mouse and keyboard plugged in to the new U2422HE, and these work nicely with the connected laptop. Just no video signal, plus the message "No USB-C signal from your device".

I'm not going to even try connecting the second U2721DE until I've sorted this bit out. Any advice, please?