Sharing this in hopes it avoids someone else spending 8 hours over 2 days troubleshooting this. All of the sudden my Rachio went offline. It would connect to wifi, but not to the Rachio Cloud (3rd light flashing constantly).
Router: Asus RT-AC87U (2.4Ghz SSID)
Router Firmware: Merlin 380.66_4
Rachio: Gen 2 8 Zone Model
What did not work:
*Rebooting router / modem
*Redoing the wifi setup on the Rachio
*Switching to Rachio guest network
*Updating Router Firmware
What ended up fixing it:
*Disabled the asus firewall just to get the Rachio online. Re-enabled the firewall after it was back online. Working fine now.