Router Eltex NTU-RG-1431G-Wac
I connected the external hard disk to the router, and I want to access it via the Internet.
For 192.168.1.1:21 I did port forwarding.
Through my public IP address, I can access an external hard disk. ftp://xx.xx.xx.xx user + password.
Since I have a dynamic IP address, I need a Dynamic DNS.

When I check at dynu.com whether port 21 is open, it says that it is not. https://www.dynu.com/NetworkTools/PortCheck

I do not understand this, I can access the router over the Internet at ftp. But port 21 is not visible.
I do not know how to connect 192.168.1.1:21 (my external hard disk) via dynu.com to the internet. Can this be done at all?

Test:
I did port forwarding for one program (KODI - player) 192.168.1.5:8080. Port 8080 is open.
I made a domain on dynu.com for KODI http://kodiusb.mywire.org
Only then can I open KODI if at the end http://kodiusb.mywire.org I enter 8080 http://kodiusb.mywire.org:8080/

Many regards Ernest

Don't you need to install an FTP server to host FTP service? Simply connecting the drive to the router and opening port 21 would not work.

You're right.
I managed to access my public address (ftp) only when I was connected to my network. I did not try other networks.
My mistake.