There is an easy fix!  

Going into the Xfinity app on your computer or device you need to check the connectivity of all your Sonos speakers, especially the Soundbar(s).  The Soundbar(s) must to connected to your 2.4 GHz Wi-Fi for the Surrounds to connect.  If the Soundbar is paused or disconnected the Surrounds will not connect.  

My One SL would not update from 14.8 to 14.10 (now 14.12 as of today, 20 September) when that update came out.  Sonos Support is still saying there are know issues.  These issues are from customers; not Xfinity.  Maria from Sonos Support got the problem fixed, albeit it took an hour on the phone and Maria remotely accessing my laptop, but once we got the connections in order my One SL updated in a matter of seconds and I could connect my One SL’s as surrounds again.  Please note the Surrounds have to be in vicinity of the Soundbar you are trying to connect to; not your Gateway.  I hope this helps all of you having this problem.