Assign a static public IP address to a PC using Technicolor TG582n

[#1]

I am wanting to assign my static public IP address to my main computer at home so that I can access it from outside my LAN via VPN and/or ssh (in Linux). I attempted to do this by logging into my Technicolor TG582n router supplied by plusnet by going to "Home Network" > "Devices" and underneath "Pick A Task" I selected "Assign the public IP address of a connection to a device" and then clicked "Edit", selected my computer and pressed "Apply". Then a message pops up saying I need to release and renew the IP address on my computer. I have tried to do this by shutting down the computer for several minutes or taking out the ethernet cable. I've also tried running "ipconfig /release" and "ipconfig /renew" in a windows command prompt but although my IP address does change, it doesn't change to the static public IP and when I navigate back to the "Assign the public IP address of a connection to a device" section it says "Not Assigned". Am I doing something wrong? What do I need to do to get it to work?

[#2]

You shouldn't need to put it in the dmz to be able to do what you want.
Just use port forwarding,
say port 333 to 22 on the machine - then you connect to <your public ip>:333 to get to it via ssh
and open the ports for the vpn (sorry never done one of them)
Mind you a lot of people (me included) have issues with these routers.
Some people have found it take a LOT of prodding to get it to forward ports (I used upnp when I used it and it worked ok.)