DIAGNOSIS: The purifier is connected to your local Wi-Fi but has lost its connection to the VeSync cloud servers. This is a common app/cloud service issue, not necessarily a problem with your device or home network.
FIX: First, restart your home Wi-Fi router. Wait 2 minutes for it to fully reboot. Then, power cycle the purifier (unplug for 60 seconds, plug back in). Open the VeSync app and pull down on the device screen to refresh. This often re-establishes the cloud handshake.
IF THAT DIDN'T WORK: Delete the device from the VeSync app. In the app, go to the device page, tap the settings (gear) icon, and select 'Delete Device'. Then, re-add the purifier using the standard setup process. This creates a fresh cloud connection.
HEADS UP: The 400S requires a 2.4 GHz Wi-Fi network. It will not connect to 5 GHz networks. Ensure your phone is also on the 2.4 GHz band during the setup/reconnection process.