Google Wifi and Nest Wifi don't have a feature to enable separate network names for the 2.4 GHz and 5 GHz bands, so there's also no option to turn off just one of the bands.
Client devices (i.e. your TV box itself) decides which of your Wifi points and which band to connect to, and also when to switch between the bands. Like most Wifi systems, Google Wifi and Nest Wifi use industry standard mechanisms (known as 802.11k and 802.11v) to help devices make these decisions. However, not all devices support those specifications, or if they do, they arguably may not implement them particularly well. If there's no option in the settings on the device to force the band, there's no way Google Wifi can force it to connect to a specific band.
I don't know where your boxes are in relation to your Wifi points, but the best thing you can do here is do things that improve signal quality in the area where your device is rather than trying to force it to connect to a specific band, that may not even improve its performance. I would also look at the layout of your Wifi points in general so they are positioned as much as possible as Google recommends to improve Wifi performance.
Where to place your Wifi devices - Google Nest Help
Also, is it possible that the speed you're seeing is a limitation of the Wifi hardware in the box? I'm assuming they don't have an ethernet port for you to hardwire them?
