Can you remember whether the RT-N12 was configured as a router or an access point?
The problem at the moment is that you have two routers, the D-Link and the Asus. Both of them are doing NAT, firewall, DHCP, etc. So you have a "network behind a network". You want a single network. There are two ways to achieve this. You can either change your Asus from "router mode" to "access point mode". Or you can leave the Asus as your router and reconfigure the D-Link as a bridge. I have no experience of the D-Link so I don't know whether that's possible but
this link apparently shows you how to set it up.
So the choice is yours. Reconfigure the D-Link as a bridge, or reconfigure the Asus as an access point.