New Xfi gateway won't show online on app/website, cannot activate or configure wifi

Recently I got an email directing me to upgrade my Xfi gateway, advertising faster uploads. Said modem quickly arrived, all seems fine. I plug it in and within a few minutes it's providing service (and a single white LED). So far so good.

Except I can't configure wifi in the Xfinity app. It shows offline. So I go activate it by scanning the QR code on the bottom. This gives a nonspecific error ("Sorry, we're having some trouble - we'll have to identify your gateway another way"), as does manual entry ("The CM MAC number couldn't be identified"). Meanwhile, the gateway is set up with someone else's wifi network/pass. But at least I have ethernet.

A day or two later I open the old "My Account" app. This one's interesting - it shows the MAC and model for each device on my account, which apparently aren't visible anywhere else. Lo and behold, the new MAC doesn't match the one Comcast shipped me, so I get on the live chat to try and explain. Long story short, it soon became clear this wasn't a real person - and whatever the computer was doing it had no real comprehension of what I was trying say. Worse, the system got very confused and deleted the old equipment off my account before I was able to return it! The "AI" also changed my network/password but only on Comcast's side, not the modem (which it also tried to restart, to no avail).

Prodding the chat agent any further feels like an exceedingly poor idea, so I'm here asking for support to get this mess sorted out - the new gateway properly linked to my account, and the old one returned. I would  appreciate not having to make an entire day trip commute to the nearest retail location - just found out the one in my town closed several years ago, so this is more than a slight inconvenience and would delay things considerably.