Get rid of menu bars in Ubuntu / Jetson Nano

I am working on an app running on a Jetson Nano which requires a main monitor output and a second monitor output. Both monitors should be full screen, with no menu bars or window decorations.

Everything works very fine, the Nano is a really fast and nice board, but can’t figure out how to get rid of Ubuntu menu bars.

In main.cpp I set up the window this way:

ofGLFWWindowSettings settings;
settings.decorated = false;
settings.resizable = false;
settings.windowMode = OF_WINDOW;

This is the way how in Windows and MacOS I normally get a fullscreen on the second screen.

Then I tried also:

settings.decorated = false;
settings.resizable = false;
settings.windowMode = OF_FULLSCREEN;

but on the main monitor I only get an upwards shift of the window’s content, and on the second nothing changes.

Anyone knows how to do this or even has a clue?