I have Google WiFi with 100+ mpbs, and Stadia says my connection is too unstable. I've taken the following steps: 1. I've tried to turn off all other devices, still no success. 2. Tested individual devices for connection speed and strength - all pass with flying colors. 3. Was able to load a game on my phone using Stadia, as well as my wife's - but still cannot connect to the Chromecast. 4. Unistalled and reinstalled the Chromecast. What am I missing?
I don't have a fix for you but wanted to let you know that I'm having a similar issue. If you have a Pixel phone you could try it on that. That's the only way it's worked for me and I have tried on a few different computers and the chromecast ultra that came with the founder's edition. Works great on the Pixel 3 though..
It looks like there are some other users having a similar problem as well.