When you purchase GO Secure Net on your home internet service, it is activated for that service only and the devices connected to it. So when your device is connected to the internet either through a cabled internet connection or Wi-Fi to that particular service, you are being protected. However, if your device is connected to another network, such as public Wi-Fi, other internet networks or mobile data, you are being exposed to online security threats, unless that network also has GO Secure Net activated.
When you are using the Smart Hub modem through a fixed internet connection and GO Secure Net is activated on that service, you are being protected from online security threats. However, if your connection falls on the backup system, meaning the 4G, you are not protected.