@nyonyo 

I would say place the router wherever you have the highest network usage and/or the most demanding client devices.

For example, if you were watching streaming TV and playing online games in the lower floor and go to the upper floor mainly for sleeping, then place the router in the lower floor. 

  @woozle Unfortunately, wherever I place the router, it can not get connected to the extenders, what a waste of money spending and "innovation technology", it is an irony that onemesh is supposed to solve the problem of coverage, but in fact, onemesh can only get connected if, and only if, the house is not too big for onemesh...which in fact doesn't need onemesh, I think backhaul ethernet (easymesh) is the only solution for this very disappointing onemesh