If you can’t get better speed into the house from your ISP, try looking at QoS (Quality of Service) at router end for improvement. Most folks are looking to reduce lag for gaming, so this article might help.

If it’s one particular device getting intermittent reception, try using another- maybe download wifi analyzer from the app store to your phone and walk around to see where dead zone(s) in Room A exactly are.

I’d also try moving router to top left of room. One problem is that the signal is not just going through the wall it’s going at it at very acute angle, meaning it’s going through a lot of wall to get to the couch in room A. Putting it in upper left corner means it will have to cross 2 walls to get to couch but signal is going directly rather than obliquely through wall(s). You may need a long cat6 cable to connect to your ISP-in point to do this, but they’re as cheap as chips (<€10).