There is no way to do that unfortunately. Client devices themselves decide which of your Wifi points and which band to connect to, and also when to switch between the bands. Like most Wifi systems, 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.
As a short term workaround, you may be able to specify which band(s) you want your phone to connect to in the Wi-Fi settings on your phone.
Longer term, this could indicate an issue with the placement of your Wi-Fi points. Google has good guidance on this here: Where to place your Wifi devices - Google Nest Help
One thing that's not explicitly stated is you should aim to place your primary Wifi point as close as possible to the centre of your home. That's because the Google Wifi and Nest Wifi mesh system uses a fewest hops approach. If one of your secondary points can get even a weak connection to your primary Wifi point, it will do that instead of going through an intermediary Wifi point that may physically be closer. That's why not installing them in a straight line, and placing your primary Wifi point close to the centre of your home really helps, along with all the other guidance in that article above.
