Unless you have the xfinity tablet plugged into the ethernet in the back of it, the only way it's going to get internet signal is from the special junk netgear router which they gave you at install. I dont understand what you dont understand... obviously the junk netgear router is specifically flashed / set to only provide internet to the xfinity tablet. Thats the way it is theres no other settings in the configuration menu which would allow you to connect to your personal router. You should go to settings and enter 4321 or the default password in backwords. This brings you to installer menu, if you dont believe that theres no other options just go look yourself. The xfinity 'tablet' runs a highly locked down version of android.
Also, the quality of the routter significantly matters. Obviously the ethernet ports on the cheap netgear router are only 100mbit while other high quality routers are gigabit.
The implication of that is, if you are subscribed to xfinity 300mbit then you cant possibly get 300mbit if it goes modem> xfinity home netgear> personal gigabit router
in this case it can only go
modem> personal >xfinity home.