DIAGNOSIS The single multi-function button and rocker switch interface is non-intuitive. There's no dedicated brightness button, it's buried in the OSD menu.
FIX Press the main button (usually the one labeled with a dot or "M") to bring up the menu. Use the rocker switch up/down to navigate to the icon that looks like a sun (brightness) or a tool (settings). Press the main button to select it, then use the rocker to adjust the level. Press the main button again to confirm and exit.
IF THAT DIDN'T WORK If the buttons seem unresponsive, you might be in a different menu mode. Hold the main button down for a few seconds to force-exit any menu, then try again. Consult the quick start guide for the specific button map for your model.
HEADS UP The button feel is often mushy. Make deliberate presses. Changing settings on the fly is clunky, so set your preferred brightness and forget it.