Most routers have separate bands, (2.4 and 5) and in this case each band is operating in a separate SSID, but those should have no impact on what IPs your devices use. All of the devices that connect to that Wifi router should be in same IP range regardless of the band they're on. And should be able to talk to each other.

I admit I have no experience with Nest Wifi Routers. Maybe it is configured in a weird way and doesn't let hosts on separate SSIDs talk to each other. — If so, you should find a way to turn that off. Then which band/SSID your devices use won't matter.