I just installed a new router into our set-up here at home and upon installing it the internet works fine and devices are able to connect to the built in wireless that the router has, but there is a problem. The set-up I have is I have a couple of computers plugged into the broadband modem and then I have the new router plugged into that and then have another couple of computers plugged into the router. The problem is that the two computers that are plugged into the router can see each other and the two computers plugged into modem can see each other but the computers in the router can't see the ones in the modem and vice versa.

What do I need to do so that they can see each other? The modem I have is a TP-Link TD-8840 and the router is a Linksys E2500.