Double NAT can be used for DDNS! Why not by a Router?

False commonn knowledge says that a Router behind double NAT can not be used for automatic DDNS. My Asus Router claims so at the DDNS config page. My modem is 192.168.0.1 and assigns via DHCP 192.168.0.100 to my router. My Router is 192.168.1.1 therefore sees WAN address as 192.168.0.100 (instead of atual public WAN address). Therefore, it can not be used maintain my DDNS address. That is the fact.

Here is another fact. My server (NAS) also made by Asus, has DDNS function. NAS is connected...

Double NAT can be used for DDNS! Why not by a Router?