Website tells me to call, phone tells me to use website

I am trying to activate my internet plan. When I go to xfinity.com/activate and follow the instructions there I get the following message: 

Sorry, we can't find this device.

Please give us a call at 1-800-xfinity.

We'll add this device and activate your service as quickly as possible.

When I call, the phone asks me if I'm trying to activate my internet, then goes through a completely automated process and ends by telling me to go to xfinity.com/activate.

What do I do to actually get a person to add the modem and activate service?