Problem solved. 
And it was so much simpler than I could have imagined. After trying "just about everything" I finally did what should have been the obvious FIRST thing to do: perform a factory reset of Chromecast.
Summary: the problem lies entirely with Google Chromecast.
After configuring Chromecast to a specific Wi-Fi network, the unit is "locked" to that network. Of course this is a feature that helps when restarting Chromecast. However, when changing networks, it is a problem.
Solution: FORGET NETWORK before trying to connect to a new network. Or perform a FACTORY RESET.
SOLUTION LINKS – GOOGLE CHROMECAST HELP:
• Change the Wi-Fi network of Chromecast
• How to factory reset your Chromecast
VIDEO EXPLANATION - YOUTUBE
• Google Chromecast 3rd Gen: How to Factory Reset
