I have 5 Nest Protect devices that went offline 3 days ago. They are connected to WiFi. I reset the WiFi settings on the Protects, and they are still locally connected and pingable. But they all still show “Offline” in the app. I also have a Nest Thermostat gen 2 which is working fine and does show connected, in the same app. No firewall rules in place. All batteries are good. No changes in the network. Just all my Protects dropped offline at the same time, while the thermostat remained fully operational.
Sorry for the late reply. I wanted to follow up and see I could be of any help. I found an article with more details. Please let me know if you are still having any trouble from here, as I would be happy to take a closer look and assist you further.