Hi @Ruseen
Technitium is a selfhosted DNS server mainly for using as a local recursive DNS server. In addition to that it can also be configured as an authorative server for hosting your domains, which is the same Mail in a Box already does. In their documentation they mention that it can also be configured as a secondary DNS server, but they don’t provide a lot of details on how to configure it like that. I also think that this thing is probably overkill, just for the purpose of secondary name server.
If you are interested in a simpler alternative, you can check out my guide on how to set up NSD as a secondary name server on a VPS. It is much more minimalistic. No fancy WebUI, no AdBlocking or other stuff you’ll never gonna need for the purpose of a secondary DNS server 