Not able to setup DDNS in the wireless router

I had the DOUBLE NAT problem with the ASUS RT-N56U connected to a Dlink Modem/Router when i was trying to use the AIDisk feature to access my USB hard drive via FTP. I couldn't use DDNS service, the router displayed this message : "This router may be in the multiple-NAT environment and DDNS service cannot work in this environment.''

I solved it by using "Bridge" type of connection on the Dlink Modem/Router, then setup the ASUS Router on PPPOE connection type using the Username & Password provided by ISP.

UPDATE:

To set Bridge connection type on your modem, you'll have to reconfigure your modem, either reset it to default or simply remove the current configuration, during configuration when you are presented with "type of network protocol" or "Connection type" such as "pppoe" , "ipoe" ,"Bridge", then select "Bridge" and continue to the following steps. If you don't know how to configure your modem, google the model of the modem you'll find a guide on how to do that. because each device has different interface.