I connected my controller to 2.4 ghz. Because my phone doesn't have acces to 5g wifi.
Basically, the lag is way more than when I play stadia on my computer using 4g cellular.
Now I can no longer switch it to bluetooth pairing with the chromecast which is ethernet connected. And neither can I change it to 5g wifi. How do I do this? I got acces to phone which got 5g acces, but, I cannot even change the controller settings.
There is probably a way to do this without resetting the controller, but I'm not able to experiment with it right now for you. You can see if tapping the controller icon in the Stadia app while the controller is on gives you any options for re-configuring the controller but if you can't then there is the nuclear option. Factory reset the controller and start setup over again the way you did the first time.
Factory reset your Stadia Controller
https://support.google.com/stadia/answer/9584135?hl=en&ref_topic=9495415