There is nothing you can do from Nest Wifi / Google Home to force devices on either bands/ frequencies.
2.4ghz travels further than 5ghz, make sure the device in question is close enough to a Nest Wifi puch to use 5ghz reliably. Check the signal strength with your phone and an app like "Wifiman" for instance.
On some client devices, Windows for instance, you can force 5ghz only. Don't know if that's the case with Amazon Firestick, ask Amazon.