I'm running Ubuntu 20.04 LTS. A few more tidbits should they prove helpful.
To download the jnlp file, direct Firefox to your_ISY_address/admin.jnlp as other means don't work (your_ISY_address as in 192.168.x.xx).
IcedTea is also available as a snap (search for "java" in Ubuntu Software).
I also installed Java (OpenJDK 14), but I don't think that's necessary.
Open Terminal and CD to Downloads (assuming admin.jnlp was saved in the default location). Then enter
javaws admin.jnlp
Ignore the desktop shortcut, as it just downloads another copy of the jnlp script.
I have a 4K TV as a monitor. Not only is the font minuscule but the logon window cannot be accessed. Once I adjusted the display resolution in Settings to 1280x720 I could add devices and modify programs, but it wasn't pretty. I'm sure there is a way to add a launch script that could include display variables, but I was just happy to add a device while my Windows machine is down.