$400 Visa Card

I received a $400 Visa card from xfinity mobile.  I activated it by calling the number on the back. It says I am activated with a balance of $400.

I've tried to make small purchases from various places and each time it tells me the card is no good.  I tried to register it on the website listed on the back of the card www.mycardintel.com/xfinitymobile.  It keeps telling me, Invalid Card details - please reenter. Please contact Customer Service at [Edited: "Personal Information"] if you need further assistance. 

So I contacted the card thing and all they could say is it has a balance of $400!  I went through everything I needed to to get this card and now it doesn't work at all.  Four purchase from four different places can not all be wrong. 

Is this a scam from Xfinity or am I just getting screwed over by Comcast? If so I'll just cancel my mobile service.

Any help would be appreciated.