Can't start an Xfinity plan at new apartment address or get help via phone

I had an Xfinity plan last year at one place I rented for a summer internship (which is why I currently have an Xfinity account). I ended that plan about a year ago when I returned to college. Now I have graduated college and have just moved in to another apartment, in the same town as that internship. I expected that it would be just as easy to get an Xfinity plan started as it was the first time, but it has not been:

When I type in my new address--which appears quite nicely and normally in the address search bar, even down to the apartment number--and click "Check Availability" for plans, I get an error message that reads, "Your order deserves a little more attention. Unfortunately, we're unable to complete your request online, but our knowledgeable representatives are available to help you right now. Call 1 (877) 338-9791." 

I'm not sure what is up with that, because I am 99% certain that the prior occupant of my apartment--who moved out less than a week ago--also used Xfinity for her WiFi. Moreover, I can tell by typing in other apartment numbers that the two other apartment numbers right next to me are also using Xfinity WiFi and have plans (as the Xfinity website tells me that there are active accounts set up at them). In my apartment, I even see a coaxial cable left that looks perfectly suited for attaching to an Xfinity modem.

 When I consequently call the 1 (877) 338-9791 number to see what is up and what to do, I reach another dead end. I press 2 for "if you are new to comcast and would like to purchase services" and then get prompted to enter my "ten digit phone number where I would like to set up services." I do that (entering the same phone number as is linked to my past Xfinity account), but then I get the message, "we were unable to validate your selection. Please try again." And the system hangs up. So I don't know what I'm doing wrong, and the phone number that is supposed to tell me what I am doing wrong won't even let me make it past the first step. 

Is there an obvious error with what I am doing? Is there a glitch because the previous customer (my guess, anyways) has only moved out so recently? Should I be selecting something different when I call the help number? I just want to purchase services in my new place. What should my next steps be?

Thank you for any help.