I was having problems finding my server. The problem seems to come from the game and not Nitrado. I give my tip that can be useful to others.
The problem you describe is not solved with your workaround; it's "bypassed". The server doesn't show in the server list; it should appear there - that you can login on the server through direct connect is something else and while it lets you play on the server; your server is still not "published" as it should be -> invisible to everyone who doesn't have the IP