Getting a Dell U2717D working with an iMac (2020)

For months, I’ve been trying to get my iMac (2020) to work with my Dell U2717D over displayport. It would work fine with HDMI via a USB-C hub. But for some reason, my USB-C to Displayport cable just wouldn’t work, despite the fact that it worked fine with my MacBook Pro.

The iMac itself could detect the monitor. But whenever I tried to use the DP input, the monitor would just show a black screen.