Strange Problem Please Help.

[jetpack101]

Details
Router: WRT120N (Firmware 1.0.0.1)
Games: 3 X-Box 360 (all Wired (CAT5))
PC: 2 Desktop Computers (all Wired (CAT5))
Internet: UPC (NTL) 30Mb Cable Broadband

I installed a new WRT120N router today and I could not get it to connect to the internet. I called Linksys and they got the problem sorted for me.

Here is where I am at the moment.
I have a Cisco EPC2425 Router Connected to my Cable Broadband

(With this I could not get Open NAT on all 3 X-box360)
I then connected the WRT120N From its internet port to the EPC2425 Port 1
(With this I got open NAT on all 3 Xbox)


I then turned on the 3 Xbox (All open NAT) but when we search for a game it won't find one. I also found that while the 3 X-Box where on I could not connect either PC to the internet.

I then reconnected all the Devices to the EPC2425 router (Removing the WRT120N completely) and while I could get a game no problem all the X-box 360 have 1 Open and 2 Moderate NAT Settings.

I have no choice but to leave the EPC2425 Router in Place acting as the Modem.

Calling Linksys,
When I called Linksys to help me get a connection to the internet I was told to clone my MAC address and change the router to 192.168.2.1 I did this and I gained access to the internet.

QUESTION:
1. How do I setup my devices so they are all connected to the internet via the WRT120N router and the only connection going to the EPC2425 is from the Internet Port on the WRT120N

2. Would it make a difference if I assign a fixed IP to all Games consoles and PC's and if so should they be numbered under 100 (DHCP Start IP Address)