Fwiw, my outage today appears to be related to a DHCP problem that my ISP was having. After power-cycling everything this eve, I could not pull an external IP from my cable modem. Called Charter and, after testing a bunch of things, the tech confirmed it was a DHCP problem on their end. Things appear to be up now and my ATA is back online.

Anyway, sorry for the red herring. Our regular internet access (web, email) was okay throughout the day despite the DHCP problem, so I couldn't understand why the ATA was not re-registering. Makes sense now, I think.