To put it simple...
If you have a router/modem combination, nothing that connects to that will be on the LinuxMCE internal network. If you get a second router to attach to your core, you must turn off its DHCP router server, and let LinuxMCE do that job.
That way, you'll have 2 access points, one external (internet only, maybe a wifi printer or SAMBA share or two if you have any of those), and one internal (LinuxMCE with all its multimedia and home automation goodness).
All devices you want LinuxMCE to interact with directly must be connected to the internal network.* Otherwise, if you don't want to change your existing function, you'll have to use static IP addresses, and may lose significant DCE router functionality.
I recommend a second router with its DHCP turned off. I actually have 3 routers, ... one external with no wifi (a MCV Vera with wifi turned off) and two internal (with their DHCP turned off).
* Note: in my setup, the core has its own Aeon Labs USB Z-wave stick. So, it talks "indirectly" with the MCV Vera for Z-wave functions. The LinuxMCE core is essentially a secondary Z-wave controller.