DIAGNOSIS The included HDMI cable or your GPU's HDMI port does not support the required bandwidth for 240Hz at 1080p. This is a cable/port limitation.
FIX You must use a DisplayPort cable. Purchase a certified DisplayPort 1.4 cable. Connect it from your GPU's DisplayPort output to the monitor's DisplayPort input. The 240Hz option will then be available in Windows Display Settings > Advanced Display Settings.
IF THAT DIDN'T WORK Your GPU may not support DisplayPort 1.4. Older GPUs may need a driver update. Also, check the monitor OSD: under 'Game Settings', ensure 'Overdrive' or 'Response Time' is not set to an extreme mode that can sometimes lock refresh rate.
HEADS UP Even with DisplayPort, if you enable HDR in Windows, it may cap your refresh rate lower. Disable HDR to test maximum refresh rate.