Go to 192.168.1.1 - default username password is admin / admin
Check the numbers on the top right - the number above is your reserved download limit and the number below is your reserved upload limit
The download limit (above) should be at least 2000 (KB) over your internet speed.
So if you're subscribed to a 4MB plnb, the the number above should be 6000+
If it is less than that, contact them and let them know you're not getting the full available bandwidth you're paying for.
I had this issue in the beginning with my upload limit, so I had them fix it.