The Maestro docs say nothing about how to turn on the 12864. I haven't actually tried "just plugging it in" but I assumed there's something to configure as it seems like everything else on Duet needs to be configured before using.
Apart from one page on how to write your own menus, this seems to be the only meaningful thing in the Maestro docs about the 12864:
The Duet 2 Maestro also support the PanelDue colour touchscreen. Connectors for a 12864-type mono graphics display and rotary encoder are also provided (these are not yet fully supported in firmware).
I haven't actually started setting up the Maestro but I've set up a DuetWifi and DuetEthernet a year ago.