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.
1. Start your game in Steam
2. Back to windows and go in steam in tab view --> Servers --> add custom server (with IP of your Nitrado server and use your Nitrado Query Port for the port!!!)
For this step, check my pictures here if it not clear for you : https://imgur.com/a/fqgufhb
3. Then put your password and click on connect (in Steam browser)
4. Then go back in the game, select as usual you the character selection then the game will launch your server and ask you to put your password.