If you turn on DMZ Hosting, then usually you don't need to port forward... (It might cause problems if you do? I don't really know.)
And sometimes it takes time to take affect or something.

I've done it a ton of ways, but it still doesn't work. When I try hosting RP Engine, for example, it appears on the hub, I can change the world status and that works, but nobody can join.

If you have your ports properly configured then there should be no reason to turn on DMZ mode.

Open a command console (Start>Run>cmd) and type in ipconfig. Make sure the ip listed there is the one your forwarding the ports to.

Try and host on a port from 3000 to 5000. Anything higher isn't really necessary, anything lower may be used by something else.

If your firewall is off and this doesn't work then there may be something wrong with the router itself. Try checking the manufacturer's site for any firmware updates or known issues.

By any chance could you tell us what you're trying to host? Some games have world/OpenPort()return hidden away, making hosting impossible.