What is your main router that your trying to connect the Linksys RE 1000 to? As in it's model number
Also. which hardware model of the Linksys RE 1000 do you have? 1.0, 1.5, or 2.0?
You might also want to try the WPS method describe on page 7-9 of this manual. But that might depend on your hardware model and if your main router can do WPS.