I bought 3 used Xfi Pods and I have the XB8 Modem/gateway and have this same problem.  I did DM, via the procedure of going to the Xfinity Chat app and saying repeatedly Agent until I got through to a Live person.  I spent about 4 hours in chat with people who went through the same steps given in the app, the result is always that the app session gets through the point of contacting the pod via Bluetooth then goes to a screen saying " on a screen with the top picture circling and says "Waiting for WiFi Extender to Come Online" and finally after 5 minutes it says " Wifi Extenders Not Coming Online". 

None of these things helped:

1. Upgrading to a higher level plan 

2. deleting the pods from your wifi details tab or leaving them there and trying to acctivate them again

3. pod is plugged in right next to the XB8 or 2 rooms away. 

4. connect the pod via ethernet cable before or after or during activation

5. Rebooting the XB8 - either by plugging and unplugging or reset by the app or a reset by the Tech Support team

6. Pulling the pod out of the wall/electricity for exactly 20 seconds and plug back in

All 3 pods behave the same way so perhaps it is an age thing and the wifi radio chips went bad in all 3 at exactly the same time - but it feels more like they are incompatible with the XB8.  The only solution technical support could give by the end was to refer to a technician visit which will be $100.  Not sure why that would help so I declined.