Poor wifi coverage with the Xfinity rented gateway

The setup in my house is fine because I have my own modem and router but recently I noticed how bad the WIFI is in my parent's house who have a rented Xfinity gateway.  It is installed in the basement at one corner of the house and when I am over there, I am usually in the living room directly above it so it is not too bad.  However, if I go to the other end of the house on the main floor or even in the basement, it does not work at all.  Does Xfinity provide free extenders or better equipment or something to increase the coverage?  What is the best way for me to handle this for them, since they won't really know what to ask for?  Thanks.