How to maintain consistent WiFi signal throughout the house?

Hello!  
We upgraded to the xFi modem recently.  The modem is upstairs, and we have a Netgear Router (extender) on the first floor to provide WiFi to the rest of the house.  Once we installed the xFi modem, and connected it to the router,  we can only get WiFi from the router and not the modem.  (before we changed modems, we could get WiFi from both devices at the same time). 

Xfinity customer service says that it needs to be either/or - that we must either connect using the router (extender), or we use the modem for WiFi, and that they both won't work at the same time  becuase there is an incompatibility between my router (extender) and the xFi modem.  They couldn't really explain what the difference is between the pods that xfinity sells and the router that I already own, since they both perform the same function.  

Can someone help?  HOW can I get wifi everywhere in the house at the same time???  Maybe I should go back to my old modem?