UMG: Navigation between different Vertical Boxes

Hello,
I prepared my Widget to be controlled by gamepad and for my first main menu (only 3 buttons in one vertical box) it works fine. Watch the screen here:

Now I want to go more specific with my little settings menu and here I have to navigate between two vertical boxes and then complete to another location (where you can see my “Back” Button).
So I tried a lot, but my UMG Navigation understanding seems very limited.

When I get into the Settings menu, my Focus starts on the Top Left Button of the topmost vertical box.
Within this VB the navigation works fine, but I cannot navigate outside of this Box. Do I have to handle Navigation for the parent? And what about when I want to go to the Back Button?

Could somebody give me an impulse to the right direction?