XB8 Gateway & Corrupted outgoing data

About three weeks ago, we upgraded to the XB8 Gateway. Ever since then, a weekly game I run has had numerous troubles.  The app I use essentially creates a web server on my PC, using port 30000, for which I have port forwarding set up in the app.  However, the people connecting to my machine are continually having files corrupted that causes graphical glitches, inability to even get to the login screen, or just purely unable to connect.  It feels like the connection is allowed, then gets shut down before everything can transfer.  Nothing about either my setup of the application nor the other player's local setups have changed, so I'm fairly confident this is tied to the Gateway in some manner.

We even had a tech come out; his tests showed some upload speed issues, so he cleaned up some extraneous splitters and an old amplifier that was incompatible, and the tests went green.  Didn't resolve the issues we're having.

I've seen some comments online that certain ports aren't "allowed" to be forwarded (with no indication in the app), but no real indication of which ones or why.

Does anyone have any other troubleshooting steps that can be taken / ideas for how to remedy this situation?