Your problem with range is likely the end devices and not router. Most routers put out the maximum legal power so the distance the signal goes is more or less the same. End devices like cell phones many times have very small antenna and run at lower power to conserve battery. It is your phone can hear the router but the router can not hear the phone.
Be very careful about the big number scam. Bigger numbers do not mean better if you can not use the features. For example almost no phone has 4 antenna so you can never use the 4x4 mimo features they use to get the higher speeds.
The best solution would be a outdoor AP that you connect via ethernet to your main router. All depends on how hard it is to get cables run and where the signal needs to get to. You can place a AP in a window but again it depends on which side of the house the windows is etc.