How to setup secondary name server

[Me.B]

Know this could be a bit obvious, but why not ask your provider to give you a secondary ip address on the sentora machine? I find that most of the times DNS providers have to have 2 ip addresses this way you could use eg: ns1.sentoramachine.local and ns2.sentoramachine.local each of these A records would point to a different ip.
Rudimentary solution but it could be useful if you don't have the real need to set up another server to serve dns queries.

Just a thought....

It's flawed by design. As we use 2 different NS to avoid having a point of failure in this case a single dns backend. This is why I always advice to use cloudflare DNS or registrar DNS instead of building a setup on only one machine ( unless you have 2 different servers).

M B