DIAGNOSIS HDR on this monitor is a software-based feature, not a hardware HDR specification. It applies an HDR tone-mapping effect to an HDR signal, but your source device must first be outputting a true HDR signal.
FIX First, ensure your content source is providing HDR. On Windows, go to Settings > System > Display > HDR and turn 'Use HDR' on. On a game console, enable HDR in the system settings. Then, with an HDR signal being received, the monitor's OSD 'HDR' setting can be toggled between On and Off. You will see a change in contrast and highlight detail.
IF THAT DIDN'T WORK The connection type may not support HDR metadata. Use a high-speed HDMI cable (labeled 'Premium High Speed') or a USB-C cable that supports DisplayPort Alt Mode. Older or basic cables may not carry the necessary Enhanced Data.
HEADS UP The HDR effect on this monitor is subtle compared to high-end HDR displays. Do not expect dramatic brightness increases. Its primary function is to prevent washed-out colors when receiving an HDR signal from a console or PC game.