An Ultra HDR file may be perfectly valid while your screen shows only its SDR base. Before judging a photograph, check the full chain: monitor, connection, operating system and viewer.
Check the display, not just the label
A monitor that accepts an HDR signal is not automatically capable of convincing highlights. Look for verifiable information about luminance, black-level control and certification. VESA publishes DisplayHDR tiers and certified products. A certificate number is not a complete measure of photographic quality, but it is more useful than a vague “HDR compatible” label.
OLED can provide deep blacks and brilliant small reflections; a good mini-LED can sustain more light over larger parts of the image. Photographic work also depends on uniformity, colour gamut, calibration options and a screen surface that lets you judge shadows clearly.
Enable HDR in Windows
In Windows, open Settings → System → Display, select the correct screen and enable Use HDR. Microsoft explains its display, connection and driver requirements. For an external monitor, check the port and cable. If the HDR switch is absent, inspect the graphics driver and the monitor's own mode too.
If you use two screens and the HDR monitor appears unsupported, try an extended desktop. Duplication can prevent HDR in some configurations. Microsoft's HDR settings guide describes further cases.
Test with a photograph
Open an Ultra HDR JPEG in Acutance Photo-View and press H. The SDR rendition should still be a finished photograph. HDR should mainly open up light sources and reflections that were genuinely brighter in the scene. If everything becomes uniformly lighter, you are seeing more than a change in highlight headroom.
Repeat the test in a browser capable of displaying gain maps, but do not assume every browser, operating system and screen follows the same colour pipeline. If you see no difference, first check that Windows HDR is active and that the original JPEG has not been recompressed.
Calibration and viewing conditions
A very bright monitor in a completely dark room can be tiring and make shadows appear too deep. Judge images under controlled ambient light and calibrate the display where possible. A file's peak target does not promise that every screen can reach it; a display with less headroom adapts the rendering to its own capability.