From https://github.com/couriersud/msigd#22-service-menu with my comments:
-Hold the joystick button down (stupid and confusing phrase, it should be "press and hold the joystick button")
-Unplug your monitor power supply - keep holding down (dude...you have already said to hold it)
-Plug in your monitor supply - keep holding down (relax man, we got it)
-When msi logo appears - stop holding down (actually MSI logo WILL NOT appear on MAG251RX, means it worked)
-When monitor displays screen - push joystick button down to show OSD ("push joystick button down" = press joystick button)
-Push joystick button down again (you will see a new menu item at the top "F" letter)
MSI - service menu/factory menu instruction
Re: MSI - service menu/factory menu instruction
That's great, thanks for sharing.
XG2431 | MSI MAG251RX | Lacie 22b4 | Sony F520