The ATVO overlay window must be at least partially visible on any monitor. If you move the overlay window all the way outside of your visual monitor space, the overlay will not be rendered and cannot be streamed by NDI. You can move the overlay almost all the way off-screen, as long as at least 1 pixel remains on screen.
So: if your monitor is 1920x1080 and your overlay is also 1920x1080, then you can move the overlay (via the ATVO settings) to for example 1919x1079. Only the top-left most pixel will be on your screen, and that is usually transparent anyway (this is fine). As long as 1 pixel of the entire overlay (even a transparent pixel) is on the screen, it will be fine.
If you move it too far, it will also disappear from the preview, so you know when you went too far.