You mentioned it would have been nice if I had shared my secret. There are no secrets to getting FTP to work. It would be nice if you shared with us what you're doing wrong. It's just as easy for you to tell us what you're doing wrong as it is for me to tell you what I'm doing right, but that you're not doing. In other words, neither of us yet knows why you're having a problem. You're trying to offer clues as to what you've done so far, and I'm trying to offer suggestions on what to check.
Once again, why are you not following-up in the same thread? That's how discussion forums work; you post a thread, and all other posts on the same topic follow below in the thread. When you post a new thread, it confuses the discussion, as parallel discussions begin to emerge, each with different investigative tracks. Work gets duplicated, tips and hints get mixed up, you start applying fixes from one thread that confuse the advice given in another thread. It's a bad way to go about getting help.
And one more thing: Patience! Just because someone hasn't posted an answer within 24 hours or 48 hours that fixes your problem doesn't mean that an answer isn't forthcoming. You began posting on a weekend, and many possible respondees are off doing their weekend thing, not sitting by the keyboard waiting to respond to your threads.
Now for more tips:
You don't have to specifically forward port 20. The FTP session sets itself up on port 21, and then the router automatically opens 20 when it's needed. You don't need to do that. Second, your FTP software doesn't need to be configured to use port 20 either. The router is listening on port 21, not on port 20. It doesn't matter whether your software is connecting from within your LAN or from outside of your LAN, it still talks on port 21.
Think of it this way: On VHF Marine radio, there is a hailing frequency; channel 9. You can call other vessels on channel 9, and even call the coast guard, harbormaster, and everyone else listening to marine radio (in the US). But as soon as you make contact, you'll say, "Please switch to 68" (or some other frequency). The FTP protocol establishes a connection on port 21, and then the server and client decide whether to use 20 also. But when they do, the router senses the server inside your network, and allows traffic on port 20 to correspond with your server (your router). This doesn't require port forwarding to be set up for port 20.
You also do not need to set up a DMZ for the LinkStation. It's not necessary, and not a good practice. Furthermore, the only ports that you need to set your router to forward the LinkStation are port 21 (FTP) and port 9000 (http/https), assuming you want web and ftp access. It is not true that the LinkStation needs a long list of open ports. That's bad information.
So, here are the issues I believe you have:
First, configure your FTP software to connect via port 21.
Second, set your router to forward only port 21 and port 9000.
I believe you're trying to connect via port 20, which won't get you anywhere.
I believe you're forwarding port 20, which is unnecessary.
I believe you're considering setting up a DMZ, or forwarding other ports not listed here, which is also unnecessary and unwise.
There you have it, those are the secrets I can share. I would have gladly done so under the first thread, which I would have checked again to follow up if I hadn't seen that you started two new threads on the subject.