So I have the following setup:

ISP Router, which is on 192.168.2.1:
-> Has NAT enabled
-> Has DMZ to 192.168.2.10

Ungangle, which is on 192.168.2.10:
-> Is in Routed Mode
-> Has Internal network as 192.168.3.1
-> Has Static IP for the Asus Router which is 192.168.3.10

Asus Router RT-AC3200, which is on 192.168.3.10:
-> Has Internal network on 192.168.1.1

My tests working so far:
-> From 192.168.1.123 (which is connected to the Asus Router) I can access 192.168.2.1 and 192.168.3.1
-> 1.1 is able to ping 2.1 and 3.1
-> 2.1 is able to ping 1.1 and 3.1
-> 3.1 is able to ping 1.1 and 2.1
-> Untangle: Forwarding 80 and 443 from External to Internal on 192.168.3.10 works fine, so I can access a web server in 192.168.1.123 successfully (obviously also forwarding from Asus Router to 192.168.1.123) using my Public IP.

I am trying to expose the Public IP on the Asus Router, my FAILED tests so far:
-> If I forward DNS port 53 from External to Internal (192.168.3.10) I lose internet access on 1.1 network
-> If I do an 1:1 Nat I also lose internet access on 1.1 network
-> While trying the 1:1 NAT, I've tried with both my External IP and the router ip (192.168.2.1) but none seem to work.

The weirdest part I cannot understand at all is why do I lose internet access when forwarding port 53 (both TCP and UDP) from External to Internal at 192.168.3.10.

What I am trying to accomplish, because I have an dynamic IP, is to use Asus's DDNS in order to point my hosts at that DNS and not via my public IP (the service is free).
While doing the 1:1 NAT I did realize that adding the IP in Untangle is a mistake since its dynamic and doesn't make any sense at all, so I think an alternative is to use an DDNS in Untangle itself... but still can't understand why 1:1 doesn't work (dispite it beeing more or less a mistake).
Is there any free/cheap DDNS for untangle?

Thanks