I might have found a way to do it without adding the router back. After finding the IPs for the Battlenet server we play on, I set up Packet Filtering as the image below shows, with a filter for 4000 and 6112 for TCP and UDP for 2 PCs. That means it requires 8 filters for 2 PCs, 12 when we have someone playing on the 3rd PC, but so far we haven't had any disconnects in the last 3 gaming sessions with this method. It's possible we've just been very lucky and haven't had that combination that causes the instant disconnect, but it would be highly unusual for 3 sessions. Before this change it was happening every game, so I'm getting more hopeful this is a fix.