"This Connection is Not Private" Error

@XfinityJamesC I've had no issue with accessing certain websites known to be safe for work or personal (.org, .edu, etc) for 7 plus years. Once I upgraded to a Xfinity approved Modem and separate WiFi Router, no one in our household can visit sites. We receive a "This Connection is Not Private" and even when we click "visit anyway" it goes to a black Xfinity page that says to download the mobile app.

This error occurs on all devices (MacBook, iPad) and on Chrome or Safari browsers. If we switch to mobile wifi, we are able to access the sites. Attempted support but they could not address the issue. This is not a device related problem but an Xfinity one. Looking for any help by any users who have gone through this. Also, clearing cache's and cookies, seems to temporarily fix the issue, but it is back several hours later. Very frustrating.  Also, clicking on "always trust" the certificate helps temporarily too.