I have an Orbi RBR50 v1 for my ~4000 square foot house. I also have an RBS50, but shut it down because my connection to that was slow and a bit unstable. I also have an RBW30, which seems to work fine. The big router is on the 1st floor and the RBW30 on the 2nd floor. The RBS50 is in the basement where there is loving space as well as my office.
I find the current network to be a little unreliable with drops throughout the day. I see others report this issue too.
For an upgrade, I am wondering if I should drop the mesh system, and just go to the Nighthawk RS700S? I'm pretty sure I will be able to connect from everywhere in the house. If not, I could always get a basic extender.
I'd look at the RBE773 system first. The RS700 is pretty awesome but it still isn't going to have the coverage of a mesh system. And 4k sq ft is pushing it with a single router solution. could it cover it? Probably but it'd need to be centrally located and your devices at the peripheral would be slower.
