DDNS in multi router setup

[Morse]

My AC56U is behind my primary router (a Ubiquity EdgeMax Lite).
I do however want to use the VPN and AiCloud functions of my AC56U.

This requires the DDNS function to be activated, but because the AC56U detects it got a private IP from my primary router it refuses to activate.

So now my question is how do I get DDNS to overcome this hurdle?
Note: my ISP hardly ever changes my IP - so if I can somehow tell DDNS my IP that would be a sufficient solution.

 

[Morse]

I actually just got this working (using the static nature of my IP lease) by adding just another fw/router in between the primary router and my AC56U

I manually assigned the same IP as my external router to the AC56U box. As it resides in the same 'private' subnet with only the additional router, this does no harm. The additional router (wich is actually a LinkSys SPA that was already there for managing my VoIP services) forwards (using its NAT capability) ports 443 and 8082 (for HTTPS and AiCloud) to the AC56U box.

The result is that on the AC56U the DDNS service 'knows' the right IP, and is able to register itself on the Asus DDNS service. From the outside all traffic on ports 443 and 8082 is forwarded to the AC56U.

The result is that my AiCloud apps on both iOS and Android actually finds the AiCloud service on the AC56U both from within the home network and from outside the network.