The first things to check are the simplest. And forgive me if I,ve made incorrect assumptions on how your setup is configured... Try switching which Ethernet device accesses the modem, and which accesses the internal network. I would assume the laptop's built in Ethernet port should be a better choice for the internal network (LinuxMCE to the MD's etc.), the USB Ethernet adapter better for external internet (USB may be a bottleneck for internal communication; video streaming, etc., but less of a barrier for internet, the bottleneck being DSL). I would also assume that the built in port would be ID'd as eth0, and the adapter as eth1 (but with udev,... who knows)
Also, I'm unclear which port you used on the wireless AP to connect to the LinuxMCE core. You should probably be using one of the (4, I'm assuming) local network ports, not the upstream port used for internet (That is the port most routers use when doing DHCP server duty). Turn off the AP's DHCP server, but allow it to USE the DHCP to connect (if that's confusing, sorry)... Also,... Just to be sure that the core is working the way you think, confirm that your core has DHCP server active, before connecting the router (use the Advanced menu option from the main menu). It is possible you set it up in the Web admin panel, but some how didn't commit those changes (in other words, restarting all the services).