Hi, with Zigbee devices there is a time out feature “Consider Battery Power Devices Unavailable” which you can set in seconds.
Is there anything available for Wi-Fi devices, or a method of knowing if a device is still connected. I believe that the Shelly H & T only send data every now and them.
The Shelly device will show unavailable if the HA integration does not hear from the device (like if the device battery is dead). I do not know of any way to adjust the timeout period.