Found the answer, the link does sort of say this but lacks specificity.
If you want to install using a FIK, it is always an 'offline' installation and you must have either:
Downloaded the files beforehand using the 'download only' option while logged in through the installer application.
Have the ISO, for which you need to be an instututional admin.
After that you can install with FIK and license file.
The reason is: file download requires you to be logged into MathWorks online somehow and just the FIK and license file don't give you that.