The wifi feature on those modems suck. They basically are on par with a $30 one you would get at a store. VPN, QOS, and other features aside, they are not designed to do the basics well like decent throughput speed. They are designed for someone who doesn't need much, just something that is simple to use.
If you are on broadband 50 and up do yourself a favour and get what your paying for by buying an after market router. I couldn't even get 50 Mbps on wifi when I last tried with the all in one. Go and get a router on sale, as they are on discount all the time. Heck even a base Asus model will be much better.
EDIT:
http://www.memoryexpress.com/Products/MX47224 for one that will help.