Hi,
after having read this
description of how to configure and use Guest Access with UniFi APs I'm in doubt that these APs are capable of dealing with two VLANs at the physical switch ports they are connected to. If they could, it would be quite simple to direct them to bridge corparate WiFi-SSID to a first VLAN and guest WiFi-SSID to a second one.
Instead it looks like they are completely VLAN-unaware (they don't support SSID to VLAN mappings) and can only be connected to switch access ports, that is to a single VLAN. Hence, guests are attached to the corporate network either - there are only "post authorization access restrictions" configurable into the APs via the controller (kind of IP filter or ACL) that prevent access of guests to IP destinations, you don't want them to have access to.
With this WiFi solution you could create a guest WiFi by not using the builtin guest access function but b(u)y adding a second set of APs + controller operating on a different SSID, Wifi channel and VLAN (and IP network) instead. In this case you would connect 'guest' APs and 'guest' controller to switch access ports configured for a guest vlan. Your Fortigate could be connected to corporate and guest vlan by either two physical ports connected to two switch access ports configured for corprorate and guest vlan respectively, or, using IP subinterfaces, by a single physical port connected to a switch port that is configured as a vlan trunk port encompassing corporate and guest VLAN. Any switch-to-switch connections would be configured as vlan trunk ports either.
Or, even better, change to a WiFi solution that supports SSID to VLAN mapping.
PT