The URL I linked above gave the command netstat -ntlp, can you please execute that?
From your desktop (minecraft client machine) are you able to telnet a.b.c.d 25565?
Right now this seems very much like a connectivity issue rather than a minecraft configuration issue. That said, since the state of your server is prone to change in ways not described here with output, we’ll have to simply tackle it slowly and methodically and hope we find it.