You need to turn on the NAT function on your router and specify the ip as the server's intranet ip address, so that you can access the server's ftp service by typing in your browser: ftp://外网ip.

Of course there are some software to realize this function, you can search online.

It is not recommended to use the above method because of the security risks.

There are other professional ways to realize this, on the one hand, it needs your router support, on the other hand, it needs a professional to configure it.

Wishing you success.