backup battery for Pro WiFi Extender

I just upgraded to Pro Internet service and received a Pro level Gateway and a Pro WiFi Extender the very next day. There was no backup battery included as it apparently needs to be ordered separately.  I used the Xfinity APP to install the new Gateway using the same network name and password as recommended if you have many devices and mobile users. The setup went exactly as described and all the devices are able to connect to the internet. The install process then prompted instructions for the next step.

The next step was to connect the extender. I followed the App instructions carefully several times using various locations for the extender. In each case, after the step to read the Q-code on the label, the extender appeared to be communicating with the Gateway but ended with a message that it had encountered trouble with an option to request "help" or exit and start over from the beginning. After a few tries from different rooms I finally clicked on I need help to be greeted by the "chatbot" who recognized "extender" and gave me the instructions that were shipped with the product. After attempting to explain to the "chatbot" what had happened it "came to the conclusion that I needed to recover either my user ID and or password since I was obviously not signed on because in order to add an extender I would need to be the primary user.  This un-intelligent AI bot just kept asking multiple choice questions where none of the answers made sense to the question I started with. This degraded over time to the point I started searching for another source.

I did not receive the battery backup for the extender because as the site stated they needed to be ordered separately but there was no way to do that online.  Can the PRO extenders function without the battery backup? Are they stocked in the Comcast stores? I would like the security against short power outages.