Try going into your modem configuration page by clicking here >>> http://192.168.100.1 click tab configuration then tab near middle of page "Reset All Defaults" click that . Test speeds again .
FWIW, I believe that you will also have to hit the "Restart Cable Modem" button after doing so as well, or if it doesn't exist, then powercycle it by unplugging from the A.C power source for about a minute.
No need to hit restart for sb6120 cable modem works both ways with router in mix . In Am i'll test without router . Could be different for yours though .
