Hi...
I feel your pain especially since Xfinity replaced my router with their combo LAN \ WiFi one. This new router allows for a private WiFi network but it also broadcasts the Xfinity WiFi that is for subscribers (pet peeve #21431 that Xfinity is piggybacking my modem, my electricity, my location to enhance their WiFi footprint but they hide behind the "convenience that you do not have to share your personal credentials with visitors" - sorry for the rant).
My Note 4 \ Galaxy Tab S would constantly want to jump on the Xfinity signal because they were equal strength and messed up my Chromecast, printing, etc.... I found a great app called "WiFi Prioritizer" that allows me to put the WiFi signals I have in an order that I need. My private network is number one and I moved Xfinity way down the bottom to where it will not "hijack" my connection yet be available if I am in a public area