How to connect NETSTAR 500 ip codecs?

Trying to connect two Netstar 500 IP audio codecs via IP over WAN (1 is in North Dakota, the other is in Atlanta, GA). DCHP enabled or disabled? Enter WAN IP address of Atlanta router as "gateway" or LAN IP address of North Dakota router? Does port forwarding codec allow OUTGOING data to pass through firewall, or INCOMING DATA to bypass firewall, or BOTH,(creating a tunnel between devices)? There's tons of port forwarding info on the net about gaming and minecraft, which is ABSOLUTELY USELESS TO ME. I need to know WHAT port forwarding IS. WHAT does it do EXACTLY? Walking me through the steps on how to port forward my minecraft game tells me absolutely NOTHING. These codecs are driving me absolutely crazy. They're not computers with an O/S and a control panel and device manager so you can climb in their guts and ASK them what's wrong. Everything is just pass/fail. And so far it's FAIL EVERY TIME. Also, what is DMZ? Is it different than port forwarding, and what does that do?