i believe this is cable internet not standard through the phone line
the router you posted comes up as a cable router
please log on to your router using its ip address to find this out:
hold windows key and r > type in cmd > type in ipconfig in the black box> then look for the default gateway> type that number into your internet browser.
this should bring up the router homepage and it should state what rate your are synced at
if synced around 15 Mbs and there are no other programs using the connection
hook the computer up with a wire to the router and rerun speedtest
there may have been and influx of customers in our area meaning the connection has slowed and will probably speed up at night.
have you been downloading alot using p2p , eg torrent, if so your connection may be being throttled
if no success
try following these instructions
http://www.computerforum.com/131398-important-please-read-before-posting.html
and run for malware/virus's
if you are synced below that number
have you been constantly resetting the router? causing a intermittent connection?this can cause sync rate to drop
either way this will be something the isp will have to sort out if sync rate has dropped by half in a few days it is a matter they need to take up as they owe you explanation as to why