Port Forwarding: How does it work?

I have been attempting to open a port to host an Arma 3 server but have been unable to. I have a logged into my router, and opened the ports, but they remain closed when I try to test the ports.

I have a rather unusual setup for my connection, I'll admit, where I use Ethernet, but its through a Powerline Ethernet Adapter. Could that be a problem with port forwarding?

I use an older router, a Motorola MT7700, I think, (Im not at home to check atm.) I run Linux Mint, it has no problems afaik.