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The problem they currently have is that the cheapish consumer grade repeater they currently have causes issues where it overlaps with the router. Devices automatically prefer the stronger network, even though its often slower.
I wont be able to convince them to spend £300+ on a good Mesh setup, but I could probably convince them to get a better AP, and then run it a cable from one of the front bedrooms to the landing balcony which is pretty much the center of the house. Does the AC-LITE need an injector? It could be easier to do if its a single cable.
Recommendation still stands.
However, I have an Edwardian place and an AP and if you've got lots of brick walls you're still going to lose 20db for each wall you go through. A strong AP will help over a weak one, but if you're going through 3 brick walls then it's still going to fall.