can this devise be installed on the DINN rail inside the electrical DB box?

Hi there! Based on the search results, the Ewelink Wi-fi Geyser Smart Switch that's R395 would be a great option for you. It's designed to be installed directly onto your DB board, replacing the existing mechanical thermostat. Being WiFi connected, you can control and monitor your geyser from anywhere via the Ewelink app. It should be a simple DIY installation and help you save on your electricity bills. A few other switches like the Geyserwise models may also work but I'm afraid I don't have details on all products since I'm still in testing - let me know if you need any other info on the Ewelink switch!