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