Unable to verify identity when adding included Xfinity Mobile line

I’m an Xfinity Internet customer and my plan includes one free Xfinity Mobile line. When I try to add the line, the website says “Unable to verify your identity” after I enter my SSN and date of birth. Both are correct, and my SSN was issued very recently. I already tried chat and text support, but both redirected me back to the website. Could an Xfinity employee help me with an alternate or manual identity verification method?