Google wifi and the devices will automatically connect to either the 2.4 or 5ghz bands depending on a number of factors. For example a nest thermostat will connect to 2.4, as that is what it requires. 5ghz has a shorter range, so if a device that should be on 5 is connecting to 2.4, it may be out of effective range of the 5ghz bandwidth.
what I did to connect my 2.4ghz only smart vacuum to the google nest is use an old tablet or phone that only had a 2.4ghz connection, connect to your nest wifi and set it up there, once done your device should automatically connect to the nest wifi at 2.4ghz. good luck.
I just wanted to check in to see if you needed any more help or clarification on this. Ashepherdson and batmite are correct, the network will automatically handle connections to the 2.4GHz or 5GHz bands as devices need. If you need any more help, please just let me know.