Most routers and phones nowadays support both 2.4 and 5GHz. 2.4GHz is used most and therefore can have a lot of interference in crowded places such as a apartment building. 5GHz is less used but reaches less far where 2.4GHz connections travel almost twice the distance.
WiFi works based on channels, with this app you can view which channel your WiFi connection uses, as well as the channels your neighbors use. If your channel is used by many others the strength and speed of your connection will reduce enormously. With Wifi Analyzer you can find out which channel best to use.
the interference between channels can be clearly seen here, if tou use channel 1 6 or 11 there will be the least interference.