ID Verification Issue

I am trying to add an Xfinity Mobile plan to my Home Internet plan. I keep going through the process of providing the IEMI of the device I am bringing as well as the phone number I will be porting over from another carrier. But when I get to the review step and am prompted to provide my DOB and SSN, I keep getting the "Unable to verify your identity. Check your details and try again." error message loop. Yes, I am entering the correct information and have had an account with Xfinity for 10 years. How can I securely verify this information so that I can continue with the sign-up process?