Range extender won't pick up new router

I recently installed an Arris SurfBoard combo cable modem and Wi-Fi router. It has two networks, and my Netgear Range Extender does not recognize the faster or the slower one. How can I correct this? This is in a 2200 square foot home. The entry point for Comcast internet is a south-facing room in the center of the house. The extender is currently placed in the middle of the house. Thanks.