DIAGNOSIS Windows is not sending a proper HDR signal, or the monitor's HDR mode isn't fully engaged.
FIX On Windows 10/11, press Win+Alt+B to toggle HDR. Or go to Settings > System > Display > Use HDR. Turn it ON. On the monitor, press the menu button. Go to Picture > HDR. Set it to 'HDR' or 'Auto'. Not 'Game HDR'.
IF THAT DIDN'T WORK Your content/game must support HDR. Also, check your graphics card settings. For NVIDIA, open NVIDIA Control Panel. Under 'Adjust desktop color settings', set Output color format to RGB with 8-bit or 10-bit color.
HEADS UP This monitor's peak brightness is 300 nits. HDR effect is subtle, not like a high-end TV. Expect enhancement, not a blinding sun.