Ah ok -- that would explain the huge performance discrepancy you are seeing. 20-30mbit per spatial stream on 2.4GHz is actually quite good especially in noisier environments.
As far as 2.4 vs 5GHz band selection, unfortunately that's a complicated topic. Ultimately it is the client's responsibility to choose the correct band to use. AP band steering only can selectively ignore a client's wishes for to associate on 2.4 but that can lead to connectivity issues in itself.
Standalone and ZD managed has CLI knobs to control band steering but to be honest I haven't seen much advantage from using those adjustments with smart phones. Other approaches are to use a separate SSID for 2.4 and 5GHz which is currently not supported by Unleashed.... or turning down the transmit power on 2.4, or disabling 2.4 if you don't need it.