If it's managed by ZD controller, in ZD you log in with Putty
You have all rights in this mode.
ruckus(config)# wlan your_SSID
The WLAN service 'your_SSID' has been loaded. To save the WLAN service, type 'end' or 'exit'.
and now you type
ruckus(config-wlan)# bss-minrate 12
It sets the minimum transmission rate of the WLAN in the BSS to 1, 2, 5.5, 12, 24 (Mbps). It may change the current value of mgmt-tx-rate and ofdm-only according to the selected value.
You can apply this per WLAN
Not sure for the standalone mode