Band steering controls can be found in the radio profile for the 2.4 GHz radio.
If you go to Configuration > Radio Profiles, clone the default template (radio_ng0) to copy the default radio settings into an editable new profile. Give the profile a name and expand Optimizing Management Traffic Settings. Then enable the steering of clients from the 2./4 to 5 Ghz band, and select the mode. By default, the mix is to balance band usage 80/20 to 5 GHz.