I assume that Nethserver has only a lan port (green), the application that published 8081 port is installed on the Nethserver and that you redirect 8081 port from your internet connection to Nethserver IP.
try this from console:
config set YourServiceName service status enabled TCPPorts 8081 access public
where: YourServiceName is a description name (without spaces and strange characters)
You can change public to private if the clients that have to connect to the application are only on the trusted networks (green network and trusted network).