In order to disable Smart DNS and revert to default one follow the steps from Smart DNS setup guide for your device until the one where you're asked to set DNS Settings (or Configure DNS) to Manual. Set it to Automatic instead.
For some devices like Xiaomi TV, it is IP Settings → DHCP, instead of DNS Settings → Automatic.
If you were using a configuration profile to enable Smart DNS then disabling that profile will revert the DNS to default.
Lastly, entering 8.8.8.8 in the Primary DNS field will do the trick as this is the default Google DNS address.
If you're having trouble reverting your DNS feel free to reach out to us through live support chat at the bottom of your screen or via email: support@livingabroad.tv.