The number of subnets you have configured thankfully has no effect on a port forward. You should be able to follow the below document pretty directly.
http://support.dlink.com/FAQView.aspx?f=OUO%2bSTPS%2ffwUYa9wu3vB%2fw%3d%3d
That said, given that it sounds like you have a high stress environment (I have never met a gaming commission that took errors lightly), it may be worth your time to reach out to the old network guy if that bridge isn't in cinders. What kind of gaming company touches your hardware, that is absolutely nuts. Good luck dude, it sounds like you inherited the s***ty end of the stick.