@RandySea2 ok... as per this thread if a place my third Deco super close to my second Deco (let's say 5cm) and boot it then this 3rd would be connected to the 2nd. Well... I tried this and no luck....

Same problem here. Even though Deco M5 is a mesh system we should be able to choose the source signal. That's seems a little basic to me. My farthest Deco unit could have a better signal if it is...