I have new internet plan and their gateway, however...

Scanning for WiFi with iPhone, I see that I am a Xfinity Hot Spot.   I do not want that.  I went to my account and disabled it.   I have read comments that it sometimes reactivates.   

 When I was upgrading I asked Sales Associate if I can use my old store bought Cable Modem, it can handle the higher speed.  She said I would have to pay for the unlimited if I used it.   

 Is this true?  I don't mind the extra cost.    She lied to me that day about another issue.  Perhaps I could make a small Faraday Cage,  Gateway is set to Bridge Mode.