I have put the question to ASUS regarding DDNS and Automatic WAN IP address update and asuscomm.com , so hopefully they will respond
DDNS is ideally needed if you are doing any form of access from the Internet unless you have a fixed (WAN) IP address.
DDNS etc. allows access (clients) to your Domain / LAN etc, enabling ‘Administration > System > Remote Access Config’ basically allows administrative traffic/access to go via WAN for Remote Config etc, where as if you was sitting on your Lan and accessed the administration control panel from administration purposes, this would be done via LAN.
A while ago, ASUS did have the issue that when using the Asus Router mobile app and DDNS service was enabled, this automatically enabled Remote Access Configuration also, that was quickly changed 
In a completely sane world, madness is the only freedom (J.G.Ballard).
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