Yes I also spent more than one whole day on this, doing exactly this, with no luck: as soon as one extender is wired, I am losing the Wifi on my mobile and it switch to 4G.
Maybe as well this may work well in router mode, but so far, maybe I am wrong, I am thinking that the router capability of this beast is not equivalent to the Dlink DIR router that I have been using for years, so I am trying to set this up in BRIDGE mode.
In the meantime I have opened a ticket. The first answer stated that backhaul is suppose to work - again maybe in router mode -so that is good to know.