Hi svl67,
This forum is for the FortiWifi product which is FortiGate with built in wifi, so this question is in the wrong place, you would want to ask it in the normal FortiGate section.
However let me address a couple of your questions here.
First, that netgear router is probably intended for home use and may not be appropriate for use in an office environment. If it is NATing (and it probably is) this will create some headaches and cause issues with things like multicast and broadcast traffic.
I would strongly suggest that you purchase a FortiAP which is an enterprise grade solution and is quite cost effective. If that is absolutely not possible, then the next best thing would be an access point (and it needs to be ONLY an access point, no router!). It may be possible to do this with the Netgear you have, but this is out of scope for these forums.
If you use a 3rd party AP, it would work like any other wired device on the 90D. I assume you have it in switch mode. If so, you would just plug the AP into the switch and it will work.
If you need more help than that, please repost this question in the correct forum.
Cheers!
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Sean Toomey, CISSP FCNSP
Consulting Security Engineer (CSE)
FORTINET— High Performance Network Security