Sent to collections-even xfinity can’t explain

This is a long post, which is partly venting and maybe someone will help?

In the process of moving, last April, I arranged via the xfinity site to terminate and start services at the respective old and new addresses. There was an overlap, intentionally for about a week.
 I saw that this had not been set up correctly, when I checked the status online, so I contacted chat. There I had a person resolve it, but it turns out they didn’t actually resolve it. I contacted Comcast and spoke with a very helpful  live person who was assigned to people who are moving. She told me that the previous person hadn’t set it up correctly, but he had given me a good deal, and that was very nice. So she set me up, one account closed (though it is still listed on my account!) and the new residence account. Late last month I received a notice of collections from Sequium, a collections agency. It was regarding my former account. Comcast had tried to reach me in the week previous but I didn’t call back, because I assumed it was Comcast trying to have me sign up for their business account, as they have tried often in the past. I went to the chat app and though he was very nice and maybe there had been a balance, though there was nothing on my account Scott this,  he sent me to an incorrect link to pay something.  I was ready to pay anything for no reason, to avoid collections, and even if I didn’t owe, because I was being tortured by the terrible phone tree and chat app. Ironically, he also was very nice. So I called the number on my phone-the number one who had been calling a few days earlier. They were Comcast, and they said they couldn’t find any outstanding bill, nothing sent to collections, and I was current. They were very sweet as well.  I tried to contact customer service at Comcast by phone. No luck. The phone and the chat are pretty much hopeless. So far, aside from the super nice people, it’s been a nightmare to go through the phone tree which ends in nothing. I called Sequium and paid the bill, because I want to protect my credit score. I think Comcast owes me 51.00 because this balance was already paid, as I understand it,  and I never received notice of an overdue bill.  My last resort is up go to the actual office, but I don’t expect any resolution. I’ve pretty much decided to cut ties with Comcast, at this point. Thanks for reading. If you have used T mobile internet, I’d love to hear how it is.