Starcraft » Starcraft Network Problems?

I bought a DI-704 router w/ 4 ports hoping to play Starcraft online with 2 computers together in the same game. I'm on cable modem connection. Both computers can play fine online on Battle.net, but only when in different games. When I try to create a game, the other computer can't join it and vice-versa. And if someone else creates a game outside the LAN, both computers can join the same game, but it is extremely slow when playing and one will usually lag out. Right when one computer lags out, the game is perfect speed. I tried disabling the firewall for port 6112(for Starcraft), and adding the computers to the DMZ - I even disabled firewall altogether yet it still has this problem. Other games such as half-life counterstrike, quake III work perfect when on the same server online. It's just like this for Starcraft & Diablo2. I was wondering if any of you can help me on this issue?

Please help.

I didn't find the right solution from the Internet.

References:-
https://arstechnica.com/civis/viewtopic.php?f=10&t=991953
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Thanks!