Setting carefully selected data rates on an AirOS WLC are important. But to change it the radio needs to be disabled. Paste this commands on the WLC command line to change data rates in a very short disruption.

config 802.11a disable network
y
config 802.11a rate mandatory 18
config 802.11a rate disabled 6
config 802.11a rate disabled 9
config 802.11a rate disabled 12
config 802.11a rate supported 24
config 802.11a rate supported 36
config 802.11a rate supported 48
config 802.11a rate supported 54
config 802.11a 11acSupport enable
config 802.11a 11axSupport enable
config 802.11a enable network