Hey! Found a fix for this on the S7 Edge. I think it did have something to do with the 5 ghz band.
1: Turn on your phone's wifi
2: Allow it to connect to your chromecast
3: Turn on Smart Network Switch
4: Allow Aggressive Switching
5: Turn on your Mobile Hotspot
6: Allow your Chromecast to connect
7: Once the Chromecast is connected to the Mobile Hotspot, Pull down menu and click to turn wifi off, wait one second, then turn back on.
You should be able to mirror and cast then.
Hope this helps. Not an engineer, can't tell you why this works, maybe there is a more simplified way. I've also heard of clearing the data and cache from the Google home app, but that stopped working for me as I'm sure this method will someday too.
But for now, enjoy!
- The Gaffler