Reset the expander to the default configuration. Reset it again to make sure it has cleared (sometimes it doesn't clear). Make sure your router is broadcasting on the same default channel as the expander. Now, manually configure the expander by going to 192.168.1.240 (don't use the button on the expander).
After you get the expander configured (first without encryption on the expander or router) and saved, unplug the expander and wait a few seconds. Then, plug it in.
If it doesn't work, unplug your router and the expander, wait a few seconds, and then plug in the router first and then the expander (this resets the DHCP server in the router and allows for the expander to be detected if it hadn't been).
If you want to make changes to the expander, add encryption or change broadcast channels on the expander first and then the router.