Try same location on top floor and see h0w signals pan out on that and lower floor. The other placement cpuld be on top of the bedroom 1 wardrobe. That would reduce one more wall between the 2 bedroom and remove walls from living area too as there will only be a single door
As suggested just try varying placements of the router first before buying anything else. Simply power it on in the test spot and using any wifi meter app on android or iphone, walk around to see how it performs. I'm sure n12 should suffice when placed properly