It is worth mentioning that in some situations Smart DNS cannot be used successfully. If your Internet Service Provider diverts the DNS while using a technology called 'Transparent DNS proxy', they intercept all DNS lookup queries (TCP/UDP port 53) and transparently proxy the results. This forces you to use their DNS addresses without being able to change them. Smart DNS might have issues.
In other cases, port 53 (DNS) may be intentionally blocked.
In such a situation, we recommend that you submit a request for unblocking to your ISP's technical support department.