FTP ports

I’ve been reading posts here about FTP and ports.

I’ve tried to change FTP port  (and forwarded it in the router) but it wount work. Only port 21 works.

If I however use same public and private port (like 8765/8765), my FTP client (Zilla and Total Commander) connects but woun’t list the maps.

Any idea  where I’m doing something wrong?

Should I use UPnP?

I found that active mode works better. Why is that?

Does port 21 work fully? If so it’s possible something is wrong with the forwarding on the router.