Find main router IP on network with multiple routers

Our ISP installed a Fiber Modem in our office. For some unexplained reason they set up a second router coected to the fiber router with an etheet cable and we can only coect to the normal router (The fiber modem is not visible on the network). I am trying to set up port forwarding on the network with no success - I'm starting to think it's because I caot coect to the router with the fiber line coming in. Can someone please explain how they set it up and what I should do about it.