I think there is a limited time that you can reconnect to the battlefield after disconnecting such as 30 seconds to 1 minute or 45 seconds although, I could be really wrong about this because I've never disconnected from the war while on the battlefield.
I'm not sure about it but even if you get disconnected from GNW, you will stay in the battle till you will die or win. Happened to my group mate, there was "Leave" icon above his head.