Custom Nameservers Not Working

I added my custom name servers in CentOS WebPanel. Also registered them with domain registrar.

NS1: ns1.oxihosting.com | IP:45.77.1.58
NS2: ns2.oxihosting.com | IP:45.77.1.58

And I added these name servers to my domain schoolapp.tk but it is showing error page.
Error: schoolapp.tk’s server DNS address could not be found.

Please help me fix it asap.

you can use this site for your dns settings testing
http://leafdns.com/index.cgi?testid=0F93BC95

Currently you have the issue that nameservers at ip 45.77.1.58 are not responding meaning that they don't have this domain loaded.
So you need to check if this domain is working properly on that dns in bind configuration.

any change in bind configuration requires that you reload named.

Is dns allowed through the firewall? Worth checking also that any router has port forwarding set up for dns as well.