Quick summary: Forwarded port works from the outside world, but from the internal network using the external IP the connection is refused.

This is a simplified situation to make the explanation easier:

I have a computer that is running a service on port 12345. This computer has an internal IP 192.168.1.100 and is connected directly to a modem/router which has internal IP 192.168.1.1 and external (public, static) IP 1.2.3.4. (The router is TP-LINK TD-w8960N) I have set up port forwarding (virtual server) at port 12345 to go to port 12345 at 192.168.1.100.

If I run telnet 192.168.1.100 12345 from the same computer everything works. But running telnet 1.2.3.4 12345 says connection refused. If I do this on another computer (on the same internal network, connected to the router) the same thing happens. This would seem like the port forwarding is not working. However...

If I run a online port checking service on my external IP and the service port it says the port is open and I can see the remote server connecting and immediately closing connection. And using another computer that is connected to the internet using a mobile connection I can also use telnet 1.2.3.4 12345 and I get a working connection.

So the port forwarding seems to be working, however using external IP from the internal network doesn't. I have no idea what can be causing this, since another setup very much like this (different router) works for me. I can access a service running on a server from inside the network both through the internal and external IP.