You need a special router that allows its wan port to be a wireless connection. This is not a common configuration. hawkings makes a router like this called haw2r1. It has a extra radio chip that it uses to connect to the remote wireless. This is a special form of repeater that has a dedicated radio to connect back to main wireless and then another radio to connect to the end clients. Normal repeaters are dependent on a protocol called WDS that you may not be able to get to work if you do not control the main router. Besides WDS repeaters greatly reduce throughput.
If the device you have is not the DSL version you can load dd-wrt on it and maybe you can use it but it depends on your requirements. You can use the 2.4g and...