[AccessD] OT - Home networking question

Folks,

I know there are several members of this group who are knowledgeable about
computer networking. We have a network in our home/offices that has evolved
over time. The basic configuration is that out of our cable modem there is
an Ethernet hub. 3 cables come out of this hub and go to routers, one in my
office in one part of the house, one in my wife's office, and one to our
wifi. These have been added as requirements came about so this probably
isn't the best configuration. At the present we don't use the network to
access machines in other parts of the system. I have several in my office
that access each other, but they are on the same router.  I do want to stay
with the wired systems to the offices.  We just added a home security system
that can be monitored over the network. It is connected to my router as it
is closest to the box. I can access the system by typing in it's IP address.
Now I am trying to access it from other points in the network and the IP is
not visible. As you can probably tell I am network challenged. I think what
I need to do is find the IP address of the security system from outside my
router. How can this be done? 

Thanks 

Doug