To add a second iSmartgate to HomeKit, you need to force the sync with iCloud.

Please turn OFF your iPhone (to force the sync with iCloud), and then reset the iSmartgate device by pressing and holding the small button for 10-15 seconds until the Wi-Fi light turns off, then wait for 2 minutes for the Wi-Fi light to turn on in BLUE.

Turn your iPhone ON again, then try setting up the iSmartgate device: