how do I increase the speed of my dns server?
my server is fast only in the US but not everywhere else.
I have created two custom nameservers ( ns1 & ns2) with enom that is pointing to my linode ip address.
That probably isn't a DNS issue. It's more likely that if you only have a server that is located in the US, there's going to be increased latency for those who are connecting to it from another country.
Sure you can! One thing a lot of customers do is create a "hidden master" and publish only our name servers as their NS records and on the glue records with the registrar, so they take advantage of the widespread CloudFlare caching network Linode subscribes to.
These docs will get you started on setting up Linode DNS as a slave to your DNS master.
If you still have questions after that document, post them here and I'll follow up.