Hello,
I setup an FTP and a HTTP server on a NAS, therefore I redirect the ports 21 and 80 to this NAS.

From the DrayTek documentation:

For example, the built-in web configurator in the router is with default port 80, which may
conflict with the web server in the local network, http://192.168.1.13:80. Therefore, you need
to change the router’s http port to any one other than the default port 80 to avoid conflict,
such as 8080. This can be set in the System Maintenance >>Management. You then will
access the admin screen of by suffixing the IP address with 8080, e.g., http://192.168.1.1:8080
instead of port 80.

I am not sure about the following:
1) I access the router with 192.168.1.1 and the NAS with the external IP or DynDNS from the internet. "Allow management from the Internet" is disabled. Why I must change the internal configurator Port? What's the reason? I guess I overlook something.

2) Telnet/HTTP is for the router configurator , FTP is for firmware updates, okay, but what about the HTTPS port in "System Maintenance >>Management" ?

Peter

if allow management from the internet is disabled, you do not have to worry about it. if it was enabled, you would not be able to access your nas on port 80 via your wan ip as this would go to the management login page.
https is for a secure login runs by default internally. you could run https for external management and disable http. that way your nas would work and a secure login.
however, most users only allow management access via the lan and vpn.

Thanks for your explanations.