Have you had good results with wireless before at the green blob with other routers at point R? If you haven't done well there with other routers, the Orbi may not be better at the green blob the way that this is set up. Not sure about stone brick walls, don't see them around here, that doesn't sound good though...they might be thick, have rebar or something, and act like concrete walls do. Also plaster can be rugged for 5GHz. signals...you can have metal in a plaster wall, as opposed to sheetrock which is usually just wood and sheetrock. Some places just aren't constructed in a way that's conducive to wireless. A lot of European houses are built to last longer than the stick built houses around here, and are made of much more solid (read: less permeable to wireless signals) materials. And your diagram shows double doors between the kitchen and living room, so it isn't clear to me what those doors are made of, as well. Are the doors open or shut most of the time?
There isn't any limit on 2.4GHz. related to 20Mbps...there is a 54Mbps limit if you've turned off WMM (look at the QoS setup screens). You know, you can look at the "Attached Devices" screen, and see what clients are connected to your router and which are connected to the satellite. As well as whether clients are connected on 5GHz. or 2.4GHz. They've added information there since the previous firmware. I find that the Orbi connects all my clients to 5GHz. except for one that only has the 2.4GHz. band.
The settings are pretty simple on the Orbi, other than turning off WMM, I can't think offhand what you might be doing that would limit you so much on 2.4GHz. Unless you're getting interference from other devices, like a microwave oven or baby monitor or an old cordless phone or wireless headphones that's operating on 2.4GHz. Lots of devices operate on 2.4GHz...so you might think about that.
I'm really curious what your results have been previously with wireless?
