In order to color grade in HDR, you need to connect your HDR monitor as an external reference display inside of Resolve, not as a GUI display. This means you will need an HDR-supported DeckLink or UltraStudio device, plugged in through PCIe or Thunderbolt.
Also, if all you have access to is a consumer display, then the only ones that can “approximate” the appropriate range necessary for accurate HDR grading are OLEDs. Particularly the last two generations of LG OLEDs (C9 and CX). But even OLEDs require calibration and lots of setup. Dado Valentic has a really great guide for getting an OLED set up for HDR grading. https://youtu.be/T-oYDNbnVXQ
Also, grading in HDR is quite difficult, actually. If you’re not already familiar with grading in Resolve, then you’ll probably want to practice grading in SDR first.
This written guide of mine here on this forum is primarily intended for those who are interested in grading 4K HDR Blu-rays in an SDR environment. Grading in HDR really is quite a complicated beast at the moment for consumers. Most consumer hardware just isn’t really good enough yet.