How can I connect an ISP modem that gives me a coaxial using a Fortigate as the entry of my network?

I have a network with 40 users and we are surviving now with a 20mbps bandwidth. Here in Peru we can choose for an assimetryc network service that uses a coaxial entry to a really bad router and i want to replace this device with a fortigate 100D that I have.

I know that I can put the Fortigate back of the router of my ISP, but I want to put the Fortigate first above all so i was looking for some adapter but I couldn't find some alternative that give me some kind of guide about my specific requirement.

I would be very gratefull if you can help me to find a solution for it.

Thanks in advance.