Unless you changed security settings, the WEP key of your 2701HG-B is in square brackets [ ] on its label.

Set the WRE54G to an IP within the network range of 2701GH-B LAN, but outside of its DHCP assigned range, netmask is usually 255.255.255.0, and gateway would be LAN IP of your 2Wire. If you have WRE54G v1 (not needed for v2) you may need to set MAC address of the 2Wire which I believe would be the one listed on the 2Wire on the Home Network tab under Wireless Settings, Access Point.

I have a 2701HG-B, but not the LinkSys (just going from reading its docs). At my boss’ home we used a Netgear powerline wireless range extender instead (which transfers the network through powerline to remote AP), since something (maybe metal duct work) was blocking wireless between floors, and a simple repeater may not have helped.