I do agree that coming from standalone, the way to make 5GHz-only / 2.4GHz-only SSIDs is not immediately obvious in the ZD UI.
As Michael alluded to, the way you do this is you define your SSIDs (for example, wifi-2G, wifi-5G, wifi-both). Then, you define two WLAN groups: "For 2.4GHz Radios", "For 5GHz radios".
Assign wifi-2G and wifi-both to the 2.4 group, assign wifi-5G and wifi-both to the 5G group.
Then in the AP or AP Group settings, you can override 5G and 2G radio WLAN group. Set the 2G group to "For 2.4GHz radios", and similarly for the 5G override.
It's really straightforward once you're used to it, but at first it does feel somewhat strange.