There are certain things you can try doing in order to figure out this issue:
* For a start, reset the router to default settings.(Turn off everything & reboot in the following order: Modem, router, all devices.
Wait a minute or two after each one.)
* make sure that you have the most up-to-date firmware installed on your router
* check you have the most recent drivers on the wireless interface on your computer.
* Also keep in mind that those bandwidth test sites aren't really definitive. You might want to try 3 or 4 different ones and keep track of your stat's and then average them out and compare. That will give you much better idea what you're actually getting for bandwidth than running the test on one site only. ( fast.com , speedtest.com, ookla.com)
* Borrow a router from your friend and try connecting it and run a speed test on it .
* Contact your ISP