on box, disable dhcp, use a static IP.
on the router is doesnt work on, check what network address its using (ex : 192.168.1.0 or 192.168.0.0) also identify the routers network address.
some routers use 192.168.1.1 some use 192.168.1.254 or 192.168.0.254 and so on and so on.
on your box assign an ip address you like to it.
for instance.
ip : 192.168.1.140 (box ip, make sure no other device on network has it)
subnet mask : 255.255.255.0 (efault subnet mask for class c)
gateway : 192.168.1.1 (change this to your routers address)
dns : 192.168.1.1 (change this to your routers address unless you specifically need a different one for your network)
but again it could be a number of things, thats just the basic, always best to use a static IP on the box, or if your router supports it, ip address reservation by mac address.
anyhoo.
i`m a little confused by what youve said, as others have said, can you connect to the box internally? can you share internally?
DMZ on dlinks is normally under ADVANCED > DMZ
then just enter the ip address you have given the box and click save / apply / ok which ever.
this will drop it in the demilitarized zone, but remember all and every port will be open.
The port I've opened seems fine as other people can share from me but its when I connect the tm600 directly to the router it doesn't allow the box to communicate with the internet.
The only option I see is using DMZ but how do I go about enabling this?
when you say other people can share from me, do you mean when you use the over box as the server?
what happens when you ask the TM600 to get an external IP? doe it display an external one?