What do you mean re-set? And is the Airport Extreme router working?
If the extender was working with another router, you need to reset the extender to factory default first. Plug a pin into the reset hole and hold it for 10 seconds until all the lights flash.
Then can refer to this link to set it up: http://www.tp-link.com/us/faq-698.html
Or refer to this video: http://www.tp-link.com/us/download/TL-WA854RE.html#videos