@ledbylight 

So there is a few things here we can help with.  First, Ethernet backhaul is engaged by default when a connection is detected on one of the Ethernet ports of a "remote" Deco unit.  So, there is nothing you need to do to activate it other than plug in an ethernet cable to the unit and the other end to the port on your wall.  For the low speeds there are a few potential issues.  1.  The ethernet that was ran was not Cat5e or higher so the speeds are limited to 10/100.  2.  The Ethernet ran could potentially be bad.  3.  The ethernet being used to connect the Deco to the wall outlet is not Cat5e or higher. And lastly, the switch may only have 10/100 ports.  Any of these 4 issues could potentially be the cause of the low speed on ethternet backhaul.   

As for the modes of the Decos  They all share the same mode so if you are using the Deco as a router (we recommend this) then you do not need to change the mode of the "remote" units.  In fact you can swap out the units and change which is the "base" or main unit and which is "remote".  Another thing to note is to make sure the firmware is always up to date. 

Best Regards