DIAGNOSIS The 5-way joystick/navigation button is a common point of confusion on this monitor. It's not broken, but the tactile feedback is very subtle. A press (to select) requires a direct inward push on the stick, not a tilt.
FIX Use the pad of your finger, not the tip. Push the joystick directly towards the back of the monitor (straight in). You should feel a soft, quiet click. To navigate the OSD menu, tilt the joystick up, down, left, or right. The 'back' function is usually achieved by tilting the joystick to the left (it often has a little arrow icon pointing left).
IF THAT DIDN'T WORK Make sure you're not accidentally pushing at an angle. If it genuinely feels stuck and won't press or tilt, inspect the joystick recess for any packaging debris or obstruction. You can try power-cycling the monitor (unplug it from power for 60 seconds) in case the OSD controller has locked up, though that's rare.
HEADS UP There's no separate power button. To turn the monitor off/on via the controls, you press and hold the joystick in for about 2 seconds. A short press will just open or select in the OSD.