I understand the frustration. I was in the same boat as you. My flatmate and I had no issues prior to 2.3.0. Yes I think Bungie have changed something their end, but we did some testing, neither of us are computer geeks but we figured it was to do with the way that your Router interacts with Bungie's servers. The signal comes in at point A is split to both parties so you can get online, B1 and B2 then when you join a fireteam, point C, the router assumes it's one and the same, hence your issue. So we figured it was the router, ours was a BT home hub 6, pile of shit, user friendly waste of space. So we changed the router, stuck £60 in each, I'm sure there's cheaper options and bought a gaming router. Easy to set up and since then no issues. We've done nothing else to the connection, no settings changes, it just works. So it was down to UPnP all along, I still think it was Bungie upgrading something. But for me that worked and no issues since.