We all know that by doing port forwarding on the router, computers from outside the network are able, on the specified ports, to access internal computers by targeting the external IP.

I'm now replacing a TP-Link router with a D-link VDSL N 6740U router, (and copied over all the settings,) and I've noticed that one thing stopped working: With the TP-link router, you could access those port-forwarded computers from within the network, using the external IP, and they would be forwarded to the relevant computers. With the new D-Link router, it doesn't work.

You might be wondering, why would you want to use the external IP and port forwarding when you're inside the internal network anyway and can just access the internal IP? One example for why this is useful: You have an iPhone app that connects to a service on an internal computer. The iPhone app knows to connect to the external IP. When we put that iPhone inside the internal network (via WiFi), it suddenly stops working, because it can't access the service from the external IP anymore.

Is it an inherent property of D-Link routers that they do not allow accessing internal servers from inside the network by targeting the external IP? Or is there a way to make it work?