I have been having an XBOXONE issue stating Teredo IP unavailable. It happened after the power went out due to a storm and when it came back on this problem was happening. I talked to Microsoft and all their troubleshooting tips have not helped. Microsoft further said they are experiencing a big volume of Comcast customers having the same issue. I have a wired connection and a TG862G gateway from Comcast. Has anyone had this issue? Can anyone help me fix it?
Since you had a power outage, I would unplug the gateway and Xbox, and plug them back in to to clear any errors and reset them. If you happened to reset them too soon after the outage the issue may have still remained.
Prior to the storm, did you set up UPnP or Port Forwarding on your Comcast gateway to connect your XBox One? If so, did you verify if your settings are still there and were not changed?
Also, that gateway is a bit long in the tooth - if you are leasing it from Comcast, you might want to contact them for a replacement via (800) XFINITY, Chat Support, or visiting your local Xfinity Store.