DIAGNOSIS The router's settings have likely been corrupted or reset to a default state. This can happen if the power flickers during a write cycle to the internal memory. The hardware is on, but the configuration that holds your custom Wi-Fi name and password is lost.
FIX You'll need to perform a full system reset and re-setup. Here's the sequence: 1) On the main router node, locate the reset pinhole. 2) With the router powered on, use a paperclip to press and hold the button for at least 10 seconds. Release when you see the light start to flash amber. This wipes it back to factory defaults. 3) You must now also reset every satellite node the same way (hold reset for 10+ seconds until amber flash). 4) Once all nodes are reset, set them up again as if they were brand new using the Linksys app. This means running through the entire setup wizard, creating a new network name and password.
IF THAT DIDN'T WORK If after the reset the app still can't find the router during setup, your modem might have been affected by the outage too. Unplug your modem for 30 seconds and plug it back in. Let it fully boot (all lights normal), then try the router setup process again.
HEADS UP To prevent this in the future, plug your main Atlas router node into a good quality surge protector or, even better, a small uninterruptible power supply (UPS). A clean shutdown during an outage can save your configuration. Also, after you get everything working again, write down your new Wi-Fi name and password somewhere—you just learned why!