i encounter this same problem, still haven't solved yet

tried using powerline, but diff phase cant use
tried wifi repeated, got better coverage but speed slow

the only method haven't tried yet is to route LAN cable thru the walls on each floors and setup wifi access point.
i think this method is the most stable and future proof method. especially for multiple storey building with thick walls.
this is also the most troublesome as you have to route the LAN cable, either thru the walls/ceiling

i dont think even 1-2k router can pass thru those thick walls 

before i proceed with the drilling and access point in diff floor plan,
i will try the powerful router solution first. lol
r u using any? any to recommend?