I found this thread because I was having the same issue with Battlefield 1. We had two people behind the same router / on the same LAN, and each could individually connect to servers, but if both tried to connect to the same one, it would fail, saying that the we were disconnected from the server.
I contacted EA, and here's what fixed it for me:
Go to your router settings or contact your ISP. Open these ports / set up port forwarding rules.
TCP: 80, 443, 5222, 9988, 17502, 20000-20100, 22990, 42127
I forwarded the ports to just one of the PCs trying to connect, it seems UPnP handles the connection for the second PC (see below).
For more information on port forwarding, see this How to Geek article: http://www.howtogeek.com/66214/how-to-forward-ports-on-your-router/
I haven't done full testing, but for what it's worth, both of my PC's have static IPs assigned by my router, and UPnP is enabled. Netgear specifically says UPnP must be enabled for two players to access the same online application.
Hope this helps someone.
