Your AP is assigned an IP address by your local DHCP server. You can "reserve" IP addresses for the MAC address of your Cloud APs, outside the scope of IPs given to clients, but you do not have direct SSH access to issue CLI commands to Cloud APs to change their WAN IP address/netmask/gateway that way.
Once you define their serial numbers (Add AP in the Cloud WebUI), when rebooted they will check our Cloud AP Registrar (APR) for the Internet IP address(s) of the Ruckus Cloud controllers they need to talk to.