Well, you need to look up a port forwarding tutorial for your router. Then, in Dolphin's options/configuration page, set a port for Netplay Listen port. Then, go into your router and port forward for that port. That should fix things, or at least make it so you can always host.
DMZ just port forwards every port to your computer. Anyway, since each router is different, I can't exactly tell you what to do.
DMZ just port forwards every port to your computer. Anyway, since each router is different, I can't exactly tell you what to do.