"While there has been various HDR enablement efforts for years, it hasn't crossed the finish line due to Wayland vs. X.Org and the multiple Wayland compositors, the various GPU driver differences, and similar challenges. Plus many open-source developers not having any HDR displays due to the associated cost."
As far as I found out (2021 12), there is no HDR support for Ubuntu yet, you can only use SDR. But the developers are working on this (see Red Hat Is Hiring So Linux Can Finally Have Good HDR Display Support):
"While there has been various HDR enablement efforts for years, it hasn't crossed the finish line due to Wayland vs. X.Org and the multiple Wayland compositors, the various GPU driver differences, and similar challenges. Plus many open-source developers not having any HDR displays due to the associated cost."