If your DLlink is capable of port forwarding ( which I'm not sure since all my experiences with routers are 3COMs which are capable of port forwarding ), then all you do is set up your DLink to accept port 22 and forward it to your Debian box's port 22.
On your debian box, set up SSH to listen to port 22 and open up firewall for that port ( if there is any ).
On 3COM 3C510 Home Gateway/router, it's configured via web browser. I had to log in as the Administrator, then go to Advanced tab and there I needed to enable port forwarding by selecting the radio box, put in my box that I wanted to forward ( such as 192.168.100.40 ), then port number. That's what my friend has.
At home, my Debian box is the one doing all the routing and gateway stuff.
If you post your model number and if DLink has manual that we can check out online, them may be I can give you specific detailed after checking the maunal out. Send me PM if you need that.