I have a Speedtouch 780 router. Inside my network is a computer that acts as a media server within the house, and as a mail server (which I use for my personal email) for outside the house. Until now, I have not been able to access the web server from within the house using the external domain name – that gave me the Speedtouch's own configuration webserver. I had to use the local IP address, or the hostname of the computer. It also meant that I had to change the server name in my mail client (Thunderbird) depending on whether I was inside or outside the house.
I have found the solution. I needed an option called NAT Loopback which is apparently only available through the command line interface, not the web interface. From a Windows (or, in my case, Linux) command line, you need to telnet to the router. Then enter:
ip config natloopback=enabled
I did not need to reboot.
Hat tip: Matt Buckett