You can change the IP address of the router to be something different than the modems IP. You'll need to disconnect the modem from the router (or the network if it's wired up some other way) and access the router config pages using the IP address. You can then change the router IP number, but just change the last number of the range, for instance, change 192.168.1.1 to 192.168.1.5. As long as there is no other device on your network with the new number, all should be well.