U2720Q, cannot access OSD menu

I cannot access the OSD menu on my new Dell U2720Q monitor.

A quick press on the front panel button closest to the power button yields Photo 2. A long press yields Photo 1. No other buttons produce effect.

I tried the unplug-power/press-power-button-for-8-seconds routine, to no avail. The menu buttons are not locked - as the Photo 1 shows, they are currently unlocked, giving me in that photo the option to lock them.