DIAGNOSIS The monitor can do 4K at 60Hz, but not 4K at 120Hz. For 120Hz, the PS5 is likely trying to send a 1080p or 1440p signal, but it might be getting blocked.
FIX First, make sure you're using the HDMI cable that came with your PS5 (it's a high-speed HDMI 2.1 cable). Plug it into the HDMI 1 port on the monitor. On your PS5, go to Settings > Screen and Video > Video Output. Set Enable 120Hz Output to Automatic. Then, under Resolution, you might need to set it to -1 (Automatic) or manually try 1080p to see if 120Hz becomes available.
IF THAT DIDN'T WORK Check the monitor's own settings. Press the joystick, go to General > HDMI Ultra HD and make sure it's set to On. This enables the higher bandwidth mode for the HDMI port.
HEADS UP This monitor's maximum refresh rate is 60Hz at its native 4K resolution. You'll only see 120Hz if the PS5 switches to a lower resolution (like 1080p), and even then, it depends on the game supporting it.