Possible fix for Chromecast and DPC3939B router/modem...

I was having the same issue when trying to connect the Chromecast to the DPC3939B router/modem. Then while futher playing around and trying to set up the Chromecast via Android (was easy to setup through my old Droid Razr Maxx I still have laying around) through the network setup screen, I choose 2.4Ghz home network which I had hidden/not broadcasting so manually had to type the name of the network in and then WPA or WEP (forgot which one) instead of WPA2 and it connected. Don't know if this option is on the iOS Chromecast app. Hope this helps you if you are having issues.