TP Link Range Extender Can't Find Router

My router is from Comcast and is relatively new. The router is also used for VoIP. The router signal is great except in the back of the house. I bought a TP Link Extender to try and up the signal. I've tried everything and the TP Link Extender cannot find my router. It can find my neighbor's router, but not mine.

I followed the direction and connected the TP Link Extender to the router via Ethernet and plugged in the Extender (had to use an extension cord). I'm sitting right in front of the router with the extender and the extender can't find the signal. I tried TP Links tether software and it can find every router in the neighborhood, but mine. Any suggestions?