Split DNS & duckdns.org

[pickone]

Hi, all!

I am trying to access my ftp server from inside the local network, but through the external ip. I already made the port forwarding, but it is not working. I manage to achieve this through NAT Reflection, but I want to do it through SPLIT DNS.
Anyone know how to achieve this, step by step?
The duckdns url is bywhonowho.duckdns.org. I added an override to unbound DNS, writing "bywhonowho" to the host and "duckdns.org" to the domain, then the IP of the computer which have the FTP open, but it is not working.

Any idea?

Thank you in advance!

[pickone]

Nobody? Split DNS and some DDNS service? :-(