On certain installations, no matter what the user tries to do with the network connection, the JNLP always attempts to start in online and throws network exception such as "No route to host: connect".
The problem is related to the currently open JDK bug which exists since version 8u172:https://bugs.java.com/view_bug.do?bug_id=8203396
A tested work-around is to have Java 10 installed, which allows viewing of the installed applications and run them offline manually. To do so:
It may be also viable to use
javaws -offline myApp.jnlp
However this wasn't tested, as the JNLP should be launched from the Java cache location, when working in Offline mode.