There is a thread in these forums outlining a problem with the Arris SB8200 modem and utter lack of support available from either Arris or Xfinity to diagnose and fix the problem. Problem rendered Xfinity's internet service unusable and I was operating from a cell phone hotspot for a week.
My problem was only fixed by buying another modem. (In my case, a refurb Netgear CM1100.)
Spend the extra $50 or whatever it is, or just get the xFi rental, but avoid this Arris product. The headache is not worth it.