Apple Mail Login

I am supporting my parents, who have used Apple Mail to access their Xfinity Mail.  They lease a modem from Xfinity and just got a replacement.  I am no longer to login for them through Apple Mail, neither on their machine nor mine. The error message is "unable to verify account name or password".  

Troubleshooting steps I have taken have included: 

-Confirming that I can login through the web app

-Quitting/restarting Mail

-Quitting/restarting Mac

-logging in to XFinity through an Incognito Window and then trying to add the account, clearing cache

-Toggling the Xfinity 3rd party app access checkbox off & on multiple times

-Deleting the old account on the Mac and attempting to add a new one. 

-Chatting with Xfinity Assistant

-Pulling my hair out after Xfinity Assistant assured me in an endless loop (both verbally and in chat!) that everything will soon be wonderful as I will be able to access my mail through Yahoo. 

I have attempted to follow Xfinity's instructions about setting up ports, SMTP & IMAP in the Mail app, but get nowhere as I am unable to add the account to do so. 

I see in both Apple and Xfinity fora that this has been an ongoing problem for Apple Mail users over years.  I would appreciate any help to solve it in my case.