DIAGNOSIS: The monitor supports 120Hz, but you need to meet specific conditions for it to be available. This is usually a cable or GPU capability issue.
FIX: First, confirm you're using a High-Speed HDMI 2.0 (or 2.1) cable. Older HDMI 1.4 cables max out at 4K 60Hz. If your cable is good, you need to enable it on the monitor itself. Use the back joystick to open the OSD, go to Menu > Display > HDMI 2.1 and make sure it's set to On. Then, in Windows Display Settings > Advanced Display, 120Hz should now be a listed refresh rate option.
IF THAT DIDN'T WORK: Your graphics card must support HDMI 2.0 or later. Most GPUs from the last 7-8 years do, but check your specs. Also, some laptops have HDMI ports wired to the integrated GPU, which may not support high refresh rates at 4K. Try a different video output port if available.
HEADS UP: Enabling HDMI 2.1 mode on the monitor can sometimes cause compatibility issues with older devices like cable boxes or consoles. If you switch inputs often, you may need to toggle this setting back and forth, which is a known hassle with this model.