Need help with Xfinity internet (cross-post from Networking Support)

I have a wireless network set up in my home using Comcast Xfinity and a DIR 615 Dlink router. I am a using a desktop computer right now that's connected with an ethernet chord to the router and the internet is working fine.
However, once in a while, the internet goes down and it says that the network is now local only and there is no internet connectivity (which i guess means that the router is working fine and the only thing I can do is transfer files around from one computer to another using the router). All i have to do is wait for about a minute and the internet would be back online. I ran a windows diagnosis when the internet was down and it said that this couldn't be solved automatically and that the problem has something to do with the DNS (which i have no idea what that is).
This always pissed me off so today i decided to take out all the wires from the ARRIS modem (which comcast installed for Xfinity) and reconnected them. I waited a little while and decided to test the network. The wired connection is fine but the wireless connection is local only and isnt connected to the internet

Can anyone help?!?!

EDIT 1: The two laptops in my house seem to have the internet working for some reason but this desktop's wireless connection isnt connecting to the internet (I have a wired and a wireless connection on this computer)