Intersting update. I found the Chromecast's MAC address in the blacklist on my router's security that also shows all the connected devices (yet it still connects to 5 Ghz!). I turned off the Chromecast andwent in there thinking maybe I needed to manually remove the MAC connection to the 5Ghz so the router wouldn't think the same device was trying to connect to 2 networks. (It was not listed as a connected device still after I turned off the Chromecast.) I removed it from the black list and was able to connect to the 2.4 Ghz network but 15 seconds later lost the connection and the 2.4 Ghz network was not shown in the available networks. Also, despite showing "full bars" on the signal strength indicator, it lists the signal strength as poor. Where as the 5 Ghz strength icon is less full (half bars) and it lists the Signal Strength as "Good" That makes a lot of sense.

I also have an signal extender on that shows up as a separate 2.4 Ghz network that's 6 ft from the Chromecast. Can't connect to that at all. Same symptoms as trying to connect to the primary 2.4 Ghz network. My best guess is that the Chromecast compared to all my smart devices including the lights on my porch farther away from the router are less prone to signal strength issues (or possibly more likely less prone to interface from neighboring 2.4 Ghz networks) and the Chromecast is just being a lot more finicky about rejecting a connection that it thinks the signal strength isn't strong enough.

In general signal strength on 2.4 is stronger than 5 Ghz as it travels through walls easier. None of this makes sense except if I assume either the Chromecast firmware has a bug or as stated, has a higher minimum signal quality requirement than all the smart devices that stay connected.