[WiFi] Need more range for my Comcast Public Wifi

I have a new Comcast installation in my repair shop and the public wifi range sucks. There is a Cisco DPC3939 B Wireless Residential Voice Gateway with no external antenna. I presume that is the Private Wifi network router. There is an Arris Touchstone Multi-Line Telephony Modem TM604G with an internal battery Backup. I presume that is the voice modem for the phones. Is there a repeater or range extender I can get to increase the range without fooling with the existing setup or would it be better to replace the existing Wifi router with a more powerful one? Fixit Shop