I won't even go into the menagerie of fixes I tried with friends and Nintendo's support rep. I have sunk HOURS into trying to get the stupid thing to work. And yes, we've tried DMZ.
The problem is this: I can get online, I can get to the eShop, I can download eShop games, I can go into game lobbies (like RE or MK7 or DoA). But the second I press A to CONNET to a game, it times out, gives me an error code, and spits me back to the main menu. NOTHING has helped fix this issue. I need a new router.