Fallback DNS works but Private Reverse does not

[ServerGuyHK]

If I put my routers DNS IP in to the "Fallback DNS Servers" entry and "Test Upstreams", all works fine.
But it I put the same IP in to the "Private reverse DNS servers", the Test Upstreams" fails with the message "Server xx:53 could not be used.

Anyone have any ideas why the server cannot be used for Private Reverse DNS,

I can use it from a command prompt via NSLookup etc and everything resolves correctly.