xfinity support

I was issued a 300$ virtual mastercard, I am the caregiver for my grandmother and she has no idea about technology so I had it sent to me. It worked for 2 transactions for a total of around 75$ now when I try to use it on any retailer it wont get accepted nor can I check the balance anywhere online. I tried calling and got nowhere with the automated service I need to talk to an actual person.