There are different ways how to setup Smart DNS. I have an account since 3 years and never had issues,
Did you setup the DNS Servers from Smasrt DNS in your router? And your devices are getting the IP via DHCP from the router?
Login into your account.. A bit down, you see "Acount Home" at the right hand site "Options". Click to options and
choose Smart DNS Proxy API. Copy the link, it looks like http://www.globalapi.net/sdp/api/IP/update/and_some_digits
Paste the link in a web browser using a device in the same local network. The output should be
Go back to your DNS Account to "Account home" check if your IP is activated. If the DNS is not green I am pretty sure you have a wrong setup in your router.
My advice: At the beginnig I had the same setup like you, I have changed my routers DNS. After my internet was very slow. I wouldn´t use the Smart DNS Proxy, DNS servers
for my PC´s, Smart phones, etc. In my router I am using the DNS Servers from OpenDNS. I gave my fire TV stick´s, Alexa, Playstation a static IP using
the DNS from Smart DNS Proxy.
Don´t forget to update always the WAN IP I am using a Raspberry and run a script for http://www.globalapi.net/sdp/api/IP/update/and_some_digits
How do you that? I guess you can not update it with any DynDNS Domain.