I know two ways, how it coul be done:
1. You can implement a Band Balancing feature to load balance the client between the 2,4 and 5 GHz. All dual-band client radios transmit probe requests on both the 2.4 and 5 GHz bands looking for an AP. The AP can see, that client has a dual-band radio. AP will then respond to the client using only 5 GHz.
This feature is set to 25/75 by default. It means, that 25% of clients connecting to the 2,4 GHz Band. You can change this ratio from the CLI:
ruckus(config-band-balancing)# percent-2.4G 25
Of course you can not be sure that your particularly MAC device will connect to 2,4 GHz.
2. The second way is: using much more Tx Power of the 5 GHz radio (5 dB compared to 2.4 GHz is a good choice). Then the client radio will identify the 5 GHz radio as having a stronger signal and connect to it.