Late 2015 iMac, external monitor and HDR

[imofean]

I've bought a 4k HDR external monitor (ASUS VG289Q) and connect it to the iMac ( Late 2015 27")
At first the external monitor is capped at 30Hz, after changing the adapter it can run at 4k 60Hz.
However, the HDR option is still grayed out on the monitor and there is no HDR option on youtube.

Does it mean that my iMac doesn't support HDR?
I'm using a third party adapter ( CE - Link ) the label read as follows: Mini display port (1.2) input, HDMI 2.0 output HDMI max 4k/60Hz

Thanks!

 

[Moonjumper]

I have the same model, unfortunately it does not support HDR. The iMac was the last model to support HDR in 2020: https://support.apple.com/en-gb/HT210980