Linksys WRT1900 and ATT Modem

I have Win 7 Pro on two HP desktop computers. With the AT&T modem connected to the main computer in the basement I created a home group on that main computer. On the HP desktop upstairs I was able to join the workgroup and everything was fine. I could print from the computer upstairs with the HP printer in the basement that is USB connected to the main computer. The computer upstairs is Cat5 cabled to the AT&T modem. However, since I needed to have a dual band router I purchased the WRT1900 Linksys router. When I hook this into the equation I cannot see the upstairs computer and it no longer will print. At this point in time ALL computers have ethernet connectivity to the internet. The AT&T modem is set to a pass through setting on the appropriate page. The problem is with the Linksys router but Linksys wants to charge me a service fee to help with the problem, which I will not pay them. The service should be free for the first 90 days and so I am well within that time frame.

Needless to say I have tried doing everything except setting the Linksys router to a bridge. Can anyone go through the page settings on the Linksys to let me know what I need to do in the Linksys settings to get this to work. I did the ipconfig /all and used the settings and I get error messages in Linksysmartwifi saying the last Ip address is not valid.

1) BOTH computers are hard wired to the ATT U-verse Modem model NVG589 so this is a WIRED issue not a wireless issue.
2) The settings on the ATT U-verse Modem is as follows:
Firewall set to Passthrough
Default Server Internal Address is blank
Passthrough Mode is set DHCPS-Dynamic
Passthrough Fixed Mac Address is blank
Passthrough DHCP Lease 0 days 0 Hrs 10 min 0 sec
3) Connection is through the telephone line
4) Leaving the Linksys Router WRT1900 disconnected and having the ATT Modem NVG589 directly wired to the main computer and the second computer ALL computers can print on the USB printer, but once the WRT1900 is hooked up the second printer no longer is seen on the wired system and naturally cannot print.
5) Both computers have Kaspersky antivirus but it is not this because it works when the 1900 is disconnected.
6) The Linksys Router shows Upstairs computer off line with the IP address starting with 9c first line and second line starts with 40
7) CONNECTIVITY FOR WRT1900
IPV4 auto DHCP with MTU set to auto
IPV6 auto enabled with 6rd tunnel disabled
8) LOCAL LAN TAB
DHCP service enabled
Static DNS 1-3 blank
Wins blank
9) ADVANCED ROUTER
RIP ENABLED
NAT DISABLED


This is all new to me and I am sorry if I failed to provide everything the first time so hopefully you know have enough info to steer me in the right direction.:)