Yeah listen servers should work. There's no port punch through, so like you said criminalcola (good name btw), otterr (and others) have to open the ports!
I need to do some further work to stabilize things a bit. I had to re-write the entire server browser back-end (and adjust my basic gamespy emulator) for NOLF 2 (For nolf1 I was able to just hex edit the server address, there were a lot more dependencies in NOLF 2 that made that impossible.)
If you're interested in multiplayer make sure to check out https://www.spawnsite.net and their discord. They host weekly multiplayer matches on weekends. (For NOLF1, probably NOLF2 as well if you ask!)