have you turned of smart setup
have you split the networks
Split the wireless networks by adding a 5 to the 5GHz network name - don't change the 2.4GHz name and everything will continue connecting as before. Your Chromecast may be switching between the two networks causing connection problems.
Is it not working on both the 2.4 and 5GHz networks? Have you assigned fixed channel numbers to each SSID?