Go to your address bar, type in 192.168.1.1
If you haven't changed password the default password will be admin. Leave the username blank. Otherwise you might have changed it and you should know what that username/password will be.
On the first page that comes up there should be two fields that say 'Static DNS 1, and Static DNS 2, and Static DNS 3'
They might be blank, but if they're not, write down what is in them, and change them to 208.67.222.222 for the first one, 208.67.220.220 for the second one, and 208.67.222.220 for the 3rd one(if there is a 3rd one).
Here is a picture showing what the static DNS boxes will look like.
They didn't change the 3rd one, but I would recommend doing it as some routers can default back to the original DNS addresses if the 3rd one is empty. Unlikely, but I've seen it happen.
Once you have clicked 'save settings' on the bottom...
You will have to flush your cache on your computer.
Go Start --> Run --> cmd --> type in 'ipconfig /flushdns' without the quotes.
From there check to see if it solved anything.