Using a Airport Express extender with Xfinity Box?

I have been having issues with my Apple TV buffering playback of video content and stopping music in the middle of the song. These issues are very new. I never had any problems streaming videos or audio in my house until recently.

Originally I had an Airport Express at the computer and another one on the lower level of my house to extend the WiFi signal to the basement. I have since move on to a Xfinitiy Gateway box with built in WiFi. So could I somehow use the AE extender to extend the signal from the Xfinity box or does it have to come from the Airport Express to the extender onto my devices?

If I can use it, how would I go about setting it up? I asked a rep with Comcast on the phone and she seemed to think I would be able to extend the signal with the Airport Express but she didn't know how I would set it up. She just said to add it to my network.

Thanks