Hi,
Let me share some knowledge related to DNS. I hope you are aware of the tool dig that comes with linux.
To check your nameservers are properly responding and to override the cache of your ISP, go for a recursive query.
dig +trace domain.com
Here you will see the query going to root name servers and then it refers to top level domains and finally it reaches your nameservers and from it resolves your ip.
You can also check if the nameservers are properly functioning by
dig domain.com @ns1.domain.com and
dig domain.com @ns2.domain.com
Thanks
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