Hi activ,
I don't have the ST585 but I do have the TG585v7 which is a Thompson variation. Looking at the options on my router the only way one could sort of do what you want is to assign the ssh port forward to the machine that you want to access, however that will not tie the connection down to one ip address. The ssh server will reject access that doesn't have an account on the target machine. Using strong passwords you should be able to allow access to selected users. I can't think of a way to do what you want. Maybe someone else can advise, but that's the best I can offer.