I go to some places which advertise free WiFi then find that the signal is weak, the connection is painfully slow or that they require you to sign in (Sometimes with facebook so that you advertise to the world where you are). I make sure that I have none of these issues and those hosts who think WiFi is an important factor for guests should do the same. (1) Look at the packages offered by ISPs and choose one which will allow let's say 25% of guests to be streaming youtube videos (streaming videos is demanding on bandwidth) at the same time. Your ISP may be able to tell you which package is right for your situation. (2) Even in one house, you cannot depend on one router. I connected the modem/ router the ISP provided to others which I bought myself. I used ethernet cables to connect from the main modem / router to the extra ones spread throughout. (3) Check for ' dead areas' and reposition aerials by unscrewing them from the router and extending them to inconspicuous places (4) I provide an ethernet connection too by using range extenders which have an ethernet port. I do this because I prefer to use ethernet over Wifi when I am using a laptop as the connection is more stable. (5) If for some reason you end up with no internet, guests should be told when to expect it back.
These are all things I learned through observing the problems with internet in places where I stayed so I make sure not to have the same problems. I am aware that there are several other solutions but that is what I use.