3 New Modem’s: Xfinity Systems say CM MAC address already exists

I’ve had Xfinity internet for 3 years with no issues, using my own equipment.  Since my internet service was upgraded, we’ve realized the old modem can’t support the new speeds and it’s affecting our work computers.  I have purchased 3 different modems over the past month from different sellers, and every time Xfinity systems give an error that the CM MAC address already exists, but whoever is trying to add the modem cannot find who has the CM MAC address.  Per Xfinity’s site, all the modems I purchased are compatible with the internet service I have currently.  Not sure if this is relevant, but all the modems are DOCSIS 3.1.

The first modem was purchased through Amazon (not refurbished), Motorola MB8600.  I tried using the Xfinity app to add the modem but it couldn’t get past the CM MAC address step so I called customer support.  They couldn’t add it and directed me to have a technician come out.  When he came to the house he couldn’t add it either, we figured it was a modem that had been returned or something and he recommended I get Netgear.

The second modem was also ordered through Amazon (different seller, not refurbished), Netgear CM1100.  App didn’t work again, customer support on the phone was getting the same error, they told me to go to the Xfinity store.  I go to the store, they are getting the same error and tell me it a tech issue.  I begin researching online at this point, because what are the odds that this happens with 2 totally different modems?  The original tech left me his card, I te texted him what happened and he responded that technicians can’t fix this, it’s most likely an account issue.  I try again 2 weeks later, calling customer support and I could tell the people I spoke with truly were trying to force add the modem to my account, I was on the phone for almost 2 hours, still no luck.

The third and final modem was purchased from my local Best Buy store, Arris Surfboard SB8200.  I went directly to the Xfinity store and guess what?  Same error.  I told the guy the back story, he was not sympathetic and told me again it’s a tech issue.  He basically said I need to get Comcast’s modem/ router combo, but when I questioned if I could connect my wifi mesh network he didn’t seem to think that would work.  I explained I have a large home and he told me to get a wifi extender.  Why should I buy a wifi extender when I just purchased an effective mesh network that works seamlessly?

We are quite frustrated with this situation and lack of effort by Xfinity to help fix it.  We are also out $50 for the one time the tech came to the house since it was my “personal device issue”.  Has anyone had this issue and been able to get a new modem added?