You're not the first person to report strange behaviour with xfinity devices. There may be a setting on the router to configure band steering. What I think happens is that the router erroneously thinks that the Duet should be in a 5ghz network when in fact it is only capable of 2.4ghz.
Your configs will all still remain after a firmware update. You're also on a recent enough firmware that the process is rather straight forward. It's also possible to roll back if needed, but I don't think you'll have any reason to.
Even though the configuration files remain after the firmware update I usually make a backup of all the files in the system folder as a precaution and it's nice to have versions to look back to as things change over time.