I have had similar issues as everyone else. I just got the chromecast today and I could get through the setup all the way to when the chromecast was trying to connect to my Verizon Router.
I have a MI424-WR Rev C Firmware: 4.0.16.1.56.0.10.14.4
I turned off IGMP Proxy - didn't work
I checked the Wireless Settings -> Advanced Security Settings -> Other Advanced Wireless Options -> Switch Network to Broadband - it was already set to Broadband
I went into Advanced -> Universal Plug and Play (UPnP) and checked the two check boxes - didn't work
I finally tried switching the Wireless Settings -> Advanced Security Settings -> Other Advanced Wireless Options Networking - Broadband over to Network (Home/Office) and I was able to connect!!!!!!
This Network setting seems to be the opposite of what other people are saying, but maybe something along the way triggered it as well. This was definitely not an easy set up.
I hope this helps.