Hello everyone who own the D2719HGF and are facing this same issue. I found this thread via DuckDuckGo.

Here's how you fix this and unlock the power button:

1. Press the Enter button
2. Press the Left Arrow button and choose the Reset option in the OSD menu
3. Press the Right Arrow button to select YES
4. Press Enter button
5. Your monitor will reset to the defaults and all options are unlocked.