I have two Deco X20 units and now realize that I should have bought 3 to give me coverage up to my third floor (I have one unit in the basement where the modem is and a second one in an office). But, I can't seem to find anywhere to buy a single unit, and don't want to buy a 3-pack at the prices I'm finding. What unit should I buy to give myself a bit more extension? I don't need a ton of speed, just want to be able to watch videos on the third floor. Should I buy something like the M4?