Problem with (free) secondary NS on VPS. How to decide?

Question

Domain Name System

I have a VPS server, set up my own domain zone, two ns, one from my server's IP, the other - I use free secondary NS.
I check the settings on www.intodns.com, everything is OK, no errors. But when I register these NS servers for another domain, the intodns service swears at my second NS.

DNS servers responded
ERROR: One or more of your nameservers did not respond:
The ones that did not respond are:
208.79.240.3

I tried different services (afraid, Roller Network, secondary.net.ua, etc.) What is the reason?
PS I configured VPS through the VESTACP panel.

Answer 1

Domain previously created?