WiFi Issues (Can't connect to iTunes, App Store, webpages, etc.)

So I've been having issues with loading webpages, iTunes and the App Store while connected to my WiFi. I notice that certain apps, such as Facebook, Instagram, Twitter, and YouTube have no problems loading its content. I can even perform Google searches as well and load up image searches through the Safari browser. However, I cannot load any webpages, whether from searches or typed in and will time out. iTunes and the App Store are completely inaccessible and will not connect to either one at all. I figured it could possibly be the OS, since my computer has no problems whatsoever, so I updated from 8.3 to 8.4 (having to connect to my computer, since my WiFi couldn't check for updates). After the update though, the same problems still persist and I recently came to find out that my iPad and my girlfriend's iPhone are having the exact same issues.

It seems as if the problem might be within my own router/modem (I have an xfinity modem that has WiFi built in), for I don't have these issues under other networks, for example, my work's internet. However, I'm not 100% sure what the problem really is and have no idea on how to fix it. I've tried resetting the modem several times, but with no luck whatsoever. Any help would be much appreciated, for this is becoming quite an annoying issue.