The failure case is when both lose power simultaneously, and the ATA reboots before the router.
(resold ATT fiber). Modem got a FW update and the modem rebooted. ATA was still on. I had forgotten about the update and went to bed and forgot to reboot the ATA. When I woke up 7-8 hours later the ATA was still unregistered. A manual reboot using ***999 got it registered again.