First let me clarify some verbiage. Anything that has wireless is not a modem, but a gateway, which is basically a router and modem combined. I don't just say this to nick pick, but because if you have a router, you don't need the other router in the gateway. They will conflict. Your three options are;
1. Trade in for, or buy, a stand alone modem. Some examples would be the Motorola SB6141 or the Cisco DPC3010.
2. Put your gateway into bridge mode. Only some gateways are capable of this, and how you do it depends on the gateway. What gateway do you have? What this does is turn off all the router functions in the gateway.
3.Put the router in AP mode. This takes all the routing function of the router, and turns it into a wireless access point(AP). You could use the new router for wireless, but all the routing (DHCP, NAT, Firewall, etc) would be done at the gateway.