When the Xfinity service guy came to give us a new modem after ours died about a year ago, he told us it would be better if we got our own router compared to using their modem/wifi router combo they had. So, I am assuming he gave us a modem that doesn't have wifi included with it? IDK..Lol
If your package from xfinity included the Internet he should have included a wireless router. I have no extra charges on my account for the wireless router they left me.
If you do not have a wireless network set up for your house, either call Comcast or go with the AirPort Extreme unit. I can not imagine not having a wireless network anymore.