I was having the problem where logging into My Account would constantly redirect me to a "We're getting your account set up" page after activating my service for several weeks. The problem ended up being that my address was not linked to my account correctly. I was able to fix this issue through the following steps:
1) I went to the Xfiniy login page and selected "forgot username".
2) I selected the "Confirm phone number" option. I received a text verification and confirmed the number.
3) I was then prompted to enter my physical address. After entering, the page said I hadn't made a username yet for this address, so I clicked continue.
4) After being prompted for an email address for the username, I entered the SAME email address that I used to make my original account with my internet order. When I did this, the website said "We found an account with this email, would you like to login and link these accounts?". I clicked yes and logged in with my existing account email.
5) After logging in, the account linked succesfully and redirected me to a now correctly functioning my account page, where I could see my bill, plan, etc.
TL;DR: When you order an internet plan online and make a new account at order time, the Xfinity website doesn't correctly link your physical address to your My Account at the time of order.