Try powerline wifi extenders.
We had/have same problem. We upgraded to Sky fibre a few years ago. The wifi from the Hub that Sky supplied was pitifiul anyway further than about 4m and was further affected by doors, walls and floors. This was in comparison to our previous router that did not lose any speed throughout the house. We complained to Sky who assured us their router was the best available but they did send a booster unit to try to help. Instead we installed powerline wifi extenders and got back to 140mps throughout the house and in my garden studio.
We recently had a problem with broadband outage due to a tree taking out the fibre line so were without a service for 14 calender days. To tied us over we bought a tplink 4g router. Surprisingly we were getting sppeds of 40-50mps via the 4g network and the one router would still project wifi to those sorts of speeds throughout the house and garden building WITHOUT using the powerline extenders. We have just switched to the new SKy apparently all sing/dancing very fast Hub (the white one) as of yesterday. We can get speeds of 140mps in the next room but we are back to 27mps download in the nether regions of the house although the upload speed is over 30mps which I wouldn't expect. I can't currently check the broadband as MySky says there is a problem with my account and they can't currently check it.
Having tried other routers and knowing what precentage throughput can be achieved, why Sky's equipment have such a shocking reduction wia wifi I do not know.
