Factory reset the devices. Set your DHCP server to send option 138 with IP address and the Fortigate's connecting LAN interface where the APs will receive the DHCP address. They need to also receive the Wireless Controllers option, DHCP 138. And it has to be a local network (same subnet). Good luck. Mine just spun until I did the factory reset. There's an article here that may be helpful:
Opinions expressed are my own and may not represent the official opinion of my employer.
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