Trouble setting up extender as access point with Xfinity modem/router

I have been unable to set up the above extender as an access point.  I have an Arris Xfinity (on the X1 platform) modem which also functions as a router.  I connected the modem/router to the extender with an ethernet cord as instructed, then attempted to connect to the extender. This brought me to the webpage to set up as either an extender or access point.  I selected access point.  After doing that I received a message telling me that a connection could not be established with the router, which is where I am stuck.  I am not sure if it's something with the Xfinity modem that prevents me from setting it up in this manner, and have been unable to find anything addressing this specific issue.  Any insight would be much appreciated.