@KodiaK 

Hi,

Please open a command prompt on your computer and then execute the command "ping ********tplinkdns.com". (replace the stars with your actual domain name)

Is the IP address shown by the ping command the same IP address that you used for "ftp://ipname"?

Given your problem description the answer is likely "no".

Now, can you judge if the IP address returned by the ping command is at least similar to your current Internet IP address (i.e. only the last few numbers are different) or if it's completely different (i.e. all the numbers are different)?

  @woozle 

The IP returned by the ping request is the same as the Internet ftp one

  @KodiaK 

But still, if you enter ftp://ip_address it works and if you enter  it doesn't work?

That really makes no sense to me. If ping returns the correct IP address for the domain name then ftp should get the correct IP address as well.

  @woozle 

Well, I did not put any ftp:// before the DDNS domain name :)

Seems this was the issue. I thought I had to access it like a normal site, not like an ftp server

So, if I understand correctly, this domain name will work even if my external IP will change?

  @KodiaK 

Yes, whenever the Internet IP address of your Archer AX20 changes the router will automatically submit the new IP address to the TP-Link DDNS server and then your domain name ********tplinkdns.com will point to that new IP address.