Cannot unlock my Dell monitor.

[Kronos]

Need to change the input cable setting but have found that the monitor is locked.
Seemingly all I need to do id hold the menu button down for 15 seconds but this is not doing it.

The monitor is a Dell U2711B.

 

[Kronos]

Turns out if anyone finds themselves in this situation you do not swap the input cable until you have unlocked the settings menu.