You need to scan the airwaves to see what your dealing with. I use inSSIDer but there is other software as well as apps for mobile devices that can scan. Apartment complex's are very difficult to deal with as far as Wifi goes because of all the signals. This is especially true on the 2.4Ghz band. If you can use the 5Ghz band that would be better as it has more channels available, but without scanning the airwaves, you can't see what channels would be best to use. A high gain antenna will most likely make things worse as it will pull in more interference. Is your problem weak signal or low throughput? How many walls are between where you are measuring signal from and your router? Are you currently using 2.4 or 5ghz?