Prepaid won't set up

I'm trying to activate a new xfinity prepaid internet modem that I purchased. I called customer service and they were of no help other than telling me to return the equipment. Why should I have to return it? I try to activate and it says your account can't be linked. I don't have an account but I do have an old one that still won't link. I try creating a new ID to be told my phone number doesn't match. This is supposed to be quick and easy but nothing but a pain.