Wrong modem listed on account

An old modem (not used since 2017) is listed on my account.  My current modem (owned by me) is working fine (and has been since 2017), but any place I check (comcast website, various xfinity apps, etc.) it says my modem is offline or can't be reached.  I believe this is the reason that the Xfinity Stream says that I'm not connected to my home wifi, when I definitely am, keeping me from watching certain channels/programs.  I can't re-boot the modem through the app, since it's trying to re-boot the wrong MAC address.  Etc, etc.

I've tried to re-activate my current modem multiple times through the Xfinity app.  It goes through the re-start following activation just fine, so I know Xfinity can see it.  It causes the correct modem to show up on my account for a little bit, but then goes away and only lists the old modem again.  The apps all say that my modem is not connected.

I tried the web chat help a couple of weeks ago.  They did have my correct modem's MAC address on file.  They supposedly did something to fix my issue, and said to wait a few minutes and it'd be corrected in the apps.  

Can somebody fix this for me?  I've seen a bunch of people with similar issues online, but I've not seen a definitive solution.