On the Fortigate , Create a new Interface and assign it to the Uplink of your internet or DMZ with a Vlan ID and Enable DHCP

Create a policy to allow outbound

On the Switch ( ours GS752TP ) that the access points plug into,  Tag the ports with the Vlan ID you created above, where your access points plug into as all as the port for the Uplink from the Switch to the Router

On your access points  ( Ours WNDAP360 ) create a new SSID and Tag these to the new VLAN ID