What's the reason you want to force it to connect to the 2.4 GHz band?
The Fire Stick 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 uses 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.
The most common reason your Fire Stick would keep connecting to the 2.4GHz band is simply because it gets a better connection in that band. I don't know where your Fire Stick is in relation to your Wifi points but it may be that it's too far away to get a good connection or there's obstacles in the way.
The best thing you can do here is do things that improve signal quality in the area where your Fire Stick is rather than trying to force it to connect to a specific band, that may not even improve it's 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
