There are 3 ways in which an AP discovers the controller
1. DHCP - option 43
2. ADP - requires layer2 connectivity
3. DNS query for "aruba-master"
Since, the AP and controller are not on the same VLAN, you can use option1 or option3 for the APs to discover the master controller IP address.
Option1: You need to configure the Option 43 on the DHCP server to the controller IP address
Option3: You need to configure an A record on your DNS server for "aruba-master" that resolves to the controller IP address
Hope it helps.