Opening ports, DMZ etc. still will not allow me to join games. However, here is something that may interest you. I found a way around this but ONLY for Battletoads. This method has worked for me 100% of the time. Here is how its done:
Find someone playing Battletoads online.
Highlight there screen name to join them.
Press a button (A) to join and immediately start repeatedly pressing the LIVE button as if you were playing Track and Field.

Don't stop repeatedly pressing the live button until you notice the status bar growing in the background indicating it may be connecting.
From this point it should continue to fill on its own and join the game.
Once I join games this way, there is no issues during game.
This ONLY works for Battletoads. I wish it worked with Killer Instinct as I wouldn't mind a quick game of track and field to get warmed up before a match.

I have posted a video of this process in hopes that it may shed light on the subject and possibly help an update or help future online games.
https://www.youtube.com/watch?v=e7krBaS4u68
(You Tube: Killer Instinct Arcade 1up Connection Lost Issues)
Thanks again.