1. Check the monitor's menu to make sure DDC/CI is enabled.
2. Update your graphics driver with one directly from the card's manufacturer.
3. Try a different connection type. HDMI works for sure. I read other posts that claimed that it did or did not work with certain cables. Dell says DDM (Dell Display Manager) supports DDC/CI on VGA, DVI, DP and HDMI, but not USB.
I have a Dell U2515H monitor which supports DDC/CI, but I couldn't get DDM to work. I got the message, "Check monitor OSD and connections to ensure DDC/CI is supported and enabled." I did check the monitor's on screen display (under "Others") and DDC/CI was enabled. I called Dell tech support and they said DDM only worked on Dell PCs (I have an HP). I thought that was kind of an incorrect answer. I have an Intel UHD 630 graphics card, which is also supposed to support DDC/CI. So, after a lot of googling I ended up at the Intel website where they had a graphics card driver that claimed to support features that OEM drivers didn't. After installing it, all features of DDM work. Perfectly!
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