Make sure that the installation has not already been completed using a different iCloud account, e.g. with the iPhone of a friend or family member.
If a friend or family member have already installed the products using their iPhone, they must send you an invitation so that you can share access to the products via the Apple Home app.
Ask your friend or family member to go into the Apple Home app setting, select the desired home and invite a new user.
If this is not the case, and it is the first iCloud account to access the installation, reboot the Wi-Fi and HomeKit settings stored in the Legrand smart gateway by doing a 10 seconds long press on the gateway power outlet button or gateway module configuration button (see "How to reboot HomeKit?").