Thank you for contacting .
May i know if the device is on the latest firmware ?
Are you sure you are using the correct DNS ?
Are you sure you used correct configurations for adding DNS to LAN settings through CLI ? Can you share ?
Might be you configured something wrong.
You can also add it through webui. Have you tried from webui ?
And have you checked on the device itself if it has internet connection ?
You can check in System>Administration>Diagnostics