I'm having this same issue with the link. It looks like the unlimited option was dropped from my account during the free data window and has not returned. Received notice I was approaching the limit this morning.
I sent a message on Twitter but no one has anwsered. The chat bot doesn't appear to work to get to a rep either.
Hello SEOJEONG and leahg1511. Thank you both for visiting and posting on our Forum. I can assist with reviewing each of your accounts and go over the Unlimited Data options with you. Please reach out to me via private message and include your full name and service address so I can assist you. To send me a private, message click my name (ComcastJoeTru) to view my profile. Then click "Send a message" and private message me.