Forward your ports, both.
http://help.ea.com/en/article/online-ports-to-open-pvz-garden-warfare-pc/
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Same house / Internet? This is only for same house/internet.
http://answers.ea.com/t5/PvZ-Garden-Warfare-PC/Unable-to-play-PVZ-Garden-Warfare-on-the-same-Server-from-one/m-p/3223443
Go into your router and try Disabling UPnP.
If that doesn't cut, try in windows too:
"services.msc"
"UPnP Device Host" -> "Stop service".
"SSDP Discovery" -> "Stop service".
control panel-> Network and Internet-> Network and Sharing Center-> Change advanced sharing settings-> "network discovery" turn off network discovery
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I don't know if it works for this game but you can try adding this argument to the launch of the game:
+clientport 2xxxx ((number between 20000-29999))
Make each computer different.
