I have been using owned Motorola SB6121 from 2014 and suddenly now Xfinity customer rep are saying that they do not support it any more.
I have been reading the threads and it says Xfinity does not support Leased SB6121.
@Richaparmar2369 wrote: I have been using owned Motorola SB6121 ...
In addition to the link EG posted which shows that the retail version of the 6121 is no longer supported, see http://mydeviceinfo.g.comcast.net/device.php?devid=365 which says:
Special Note on this Device
Both leased and retail versions of these devices are End of Life (EOL) EOL’d and will no longer be activated on our network. Existing network devices are not affected.
Your modem will probably continue to work for a while, but upgrading would be a good idea.