2dBi vs. 5dBi vs. 9dBi Omni Antennas
dbi 360
DBI
dbi
What is the difference between 5 dBi, 7dBi, 12 dBi and 15 dBi antennas?
Well, the higher the dbi, the wider the coverage. Based on my own limited understanding, the higher the dBi, the weaker the signal. The signal will fluctuate as it covers more areas. As for TP-Link claimed on achieving 56km, it is almost impossible unless you have a clear line of sight.
It is a better idea to buy a Range Extender, you can read more about range extender. As shown in the image below, an antenna with low dBi has 360 degrees coverage whereas a high gain antenna has more distance. It will be a good idea to purchase 1 high gain 9dBi antenna for your router.