Connecting Wireless Router to Firewall - Please Help!!!

Hello,

Have a bit of a problem I thought you good folk here could help me with. I'm good with IT but networking isn't one of my strong points let's say.

Anyway, I'm working for a company and I have been tasked with hooking one of their internet connections up to a firewall, a FortiGate 60d.

We use the WiFi from the ISP router, so in an ideal world I understand I would configure the FortiGate to act as the router which it can do and the ISP router used as an access point.

Internet --> FortiGate--> ISP Router --> Internal Network

However the FortiGate has no more ethernet ports available save for WAN 2 and the DMZ port, so as far as my knowledge goes I'm not able to connect things this way.

So, I have to set things up in the following way as I understand:

Internet --> ISP Router --> FortiGate--> Internal Network

Is there a way to do this with what I have available and what settings do I need?

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Thanks