On of the biggest issues with poor WiFi connection is conflicting WiFi channels. In Seoul, WiFi isn't as prevalent as it is in the State and that allows less conflict when it comes WiFi channels and signals.
Most routers default to channel 1, 6, or 11, but if your neighbors are all on channel 6, then you might be seeing slow speed and lower range due to interference. Try one of the other channels (start with 1, 6, and 11, but other channel numbers are worth a shot, too), to see if you get a better signal with them. Remember that your neighbors may also replace their equipment at some point, so if you see a sudden drop in range or speed, try a new channel.
