E2009W, unlock according to User's Guide doesn't work

Hello,

I got this older Dell monitor (E2009W) for free. But I am not able to unlock its OSD, not even to change video source from DVI to VGA.

I tried to hold the menu button for 15 seconds (and longer) multiple times, as it is written in the online E2009W/E2209W User's Guide. But it doesn't work

I have also tried some tips from this thread. But nothing works!

Any other tips, please?