It might simply be interference with other nearby Wi-Fi, have you tried changing the channels? 1, 6 and 11 are usually the recommended ones. See https://www.metageek.com/training/resources/why-chan...
Having a repeater will not be helping in a small area if the problem is interference.
Also given the legal requirements for power levels etc and that they are all using one of several comma chips, there is less difference between routers Wi-Fi range than we might assume from marketing and anecdotal evidence. Usually the difference is in the pattern radiated from the antenna favouring someones property layout over another one, but simply moving or turning a Wi-Fi router might achieve the same thing.
Also don't forget the Wi-Fi router is only half the story, it doesn't matter how elaborate the antenna are sticking up from the go-faster casing if the issue is receving a clean signal from your other devices.