First, enable at least two (2) SSIDs on the band that will broadcast the guest network. If you want a guest network on both the 2.4GHz and the 5GHz band, then set up at least two (2) SSIDs on the 2.4GHz band and two (2) SSIDs on the 5GHz band. You can set up the SSIDs on the basic wireless settings option on the 2.4GHz and 5GHz wireless settings pages.
Click on Guest Network and go to Selection, where you can select which SSIDs on each band (2.4GHz and 5GHz) that you would like to use as your Guest Network.
Next, setup a DHCP server for your Guest Network and Click Apply