The Game Server is trying to set up a connection. It can hear, but the client can't.
Game Server---> UPnP Router---> Public Cloud <---- NAT Non-UPnP Router <---- Game Client
So then the connection error i'm having is likely in my code and not in a closed client router port?
The client simply needs to connect to the IP/port of the listed server, and no NAT discovery or punch-through is needed.
it can't be a programing error because any client with UPnP router OR with a client machine directly connected to the internet with no router is able to hear the Game Server say "Yes, you may join the game".