[My level of knowledge with vps , linux and bind dns settings are at learning/begginer stage]
Added domain/PTR plugservice.com to droplet with ip 184.108.40.206.
Then somehow setup a vanity nameserver named ns1/ns2/ns...
The issue is solved. It was due to missing A records for NS1/NS2/NS3 vanity nameservers in the droplet's DNS manager.