Were you connecting to the 2.4ghz or the 5ghz?

Also, what speeds do you pay for? Teams is sensitive to upload speeds and xfinity has specific routers (theirs included) that are part of next gen that get the increased uploads. The SBG8300 doesn't. 

Another thought is SIP ALG could be causing issues. You could try disabling that. 

I used both 2.4ghz and 5ghz. I also used Ethernet cable and had same issues with all of these options. I have 1Gb speed and literally less than 10 ft away from the router. I even disconnected VPN and still the same problem. Currently paying $15 a month for the rental device from Xfinity. Paid over $200 a year ago for SBG8300 and it would be nice being able to use it. 

I did not try SIP ALG. 

did you test teams with anyone other than a co-worker? 
Or tried a different device? 

VPN’s are great at causing issues and are much harder to troubleshoot. 

Even with 1 gig speeds, you’re upload would only be 40mbps on xfinity. Which should still be plenty but things like teams and zoom are sensitive to the upload speeds, not the download as much. 

I did test it with external clients. I made my IT department to send me a new laptop and had the same issue. My IT department installed new drivers for Teams and still the same issue. When I using Teams Classic, the older version of Teams, the connection was stable. But Microsoft Teams discontinued the Classic version. 

I think either a new Micorsoft Teams initiated the issue or a network update from my organization. I used the device for six months without any problems. 

Interesting that teams classic worked fine but not the newer version. 

A quick google shows a bunch of users with issues going from classic to the newer versions with it not working. 

Also interesting that a router change fixed it. Wonder what handshake the new teams is requiring. 

UPnP turned on? 

I do not know about UPnP.