If your device is offline on Google Home app, it usually means Google Assistant hasn't received your device's states.
Your smart home action must use the API of reportStateAndNotification to update your device's data in the Home Graph, see Method: devices.reportStateAndNotification. You can use Home Graph Viewer to check whether the device states have been correctly reported.
In addition, your fulfillment should handle QUERY intents and return the up-to-date state of devices.
