Xfinity only assigns a single public ip address per device/modem. So while the sb8200 has more than 1 ethernet connection, only 1 device can be connected to it at a time and work. And if that address is being assigned to your pc, the router/mesh system won’t work. 

So disconnect the pc from the modem, power it off (the modem and router) for 5-10 minutes, then power the modem on and give it a couple minutes to fully boot. Then connect the router/mesh system to port 1 and power the router on. Go through its internet connection setup process. It should work fine then. 

Just keep in mind, on xfinity, you can only have 1 device connected to the sb8200. 

Thanks for the info and suggestion.  I did all of that and the wireless mesh router still doesn’t connect.  So I went to the xfinity store and got one of their newest gateways and installed it to troubleshoot my modem or router.  After putting the new gateway in bridge mode, i tried to connect the mesh router with no joy.  So my assumption is that the modem is good and the router is not working.  I will possible do additional work to see if the router can be brought back to a functional state, but this will be my last update.  

What mesh system do you have? some don’t play nicely with double nats and xfinity tends to give out modem/router combo devices. 

is the SB8200 ethernet showing up as connected when the mesh is hooked up? 

Google Nest WiFi.  It has been working with xFinity for 2 years.  Just doesn’t anymore.  I may go through a complete factory reset and then try it again.  However, I’m limited on time right now so this will have to wait a few weeks.  

I’d try the full reset on both the google and the SB8200. 

Just make sure to only have the google connected tot he modem. 

Good luck!