DIAGNOSIS Windows is not detecting a high-bandwidth display mode. This is caused by using a non-compliant HDMI cable or an incorrect GPU driver setting.
FIX First, confirm you are using an HDMI cable rated for at least HDMI 1.4 or higher (HDMI High Speed). Standard, old HDMI cables cannot carry the bandwidth for 1920x1080 at 100Hz. Second, access your GPU control panel (NVIDIA Control Panel or AMD Adrenalin). Navigate to Change Resolution. Under 'Refresh rate', select 100Hz directly from this panel, not Windows settings. Third, on the monitor OSD, ensure Game Mode is enabled, as this can unlock higher refresh rate options.
IF THAT DIDN'T WORK If you are using an HDMI switch, splitter, or docking station, bypass it and connect the monitor directly to your GPU. These intermediary devices often cap the output signal to 60Hz.
HEADS UP The S36GD's 100Hz mode is only available over HDMI. It does not support DisplayPort. Verify your system is not using a generic PnP monitor driver; install the latest drivers for your specific graphics card.