bought and activated new modem. Getting error: NET::ERR_CERT_COMMON_NAME_INVALID

I bought and activated a new modem (Arris SB6190) a few days ago.  Ever since, I will occassionally get blocked from certain webpages due to because "connection is not private".  Also I use a VPN for work and it sometimes fails to connect due to a "security" issue.

see the screen shot below for what I'm talking about.  (EDIT: don't know why the image won't load, so go here: https://www.dropbox.com/s/49cp337hngcw1qg/bad%20certificate.png)

If I click Advanced -> proceed anyway, I get redirected to the Xfinity device activation page.  It is as if Xfinity temporarily fails to recognize my modem and intercepts my internet traffic.  After I re-activate my device, the original website will successfully load, but then the problem will reemerge a few hours later.  I've probably gone through this process 10 times by now.  I've also tried rebooting my modem several times by now.  And I still get this problem several times a day.  Also, if I do nothing and don't try reactivating the device, usually the problem will eventually go away on its own.  Only to recur later in the day.