How to set up a firewall for dynamic IP access?

Question

iptables

There is a piece of iron that should put data on ftp, but this does not work if you set a password - i.e. in fact, it can only work with ftp anonymously. An error in the firmware, there is no new one and is not expected. IP is dynamic, so just taking and restricting access to FTP by IP will not work. On all possible range of the provider too it would not be desirable.
What are the options for restricting access to ftp in such conditions? I am considering an option with any degree of cycling and insanity.
Well, as an example, for seed - ddns and access by domain name? Maybe something like this? However, I don't know how to implement it either.

Answer 1

VPN use