I have a different setup with an external ADSL router.
and a second firewall for the wireless AP.

I set my gateway on the desktops to the ADSL router.

To access the AP I have a static route on the ADSL router pointing back to the Second firewall which is connected to the Wireless side.

So any requests for the AP's IP address is directed back to the AP firewall.
Clumbsy but it works.
I have had to set the gateway of the wireless' firewall to be out to the wireless network for other reasons.

Maybe for you a static route out to the wireless AP will work (showing my ignorance) Help.....me...... ops: