Offline install of latest SSDT with Visual Studio Professional 2015

Apologies for the long essay below...

The server we are using cannot have Internet access at this moment in time (or any time in the future I imagine), so I downloaded SSDT from the Microsoft website which says it's backwards compatible and followed the instructions:

https://docs.microsoft.com/en-us/sql/ssdt/download-sql-server-data-tools-ssdt?view=sql-server-2017#ssdt-for-vs-2017-standalone-installer

However, using their instructions it installs Visual Studio 2017, then installs the SSDT onto this version, when it's on the setup screen it only gives the option to install onto Visual Studio 2017. I tried on the next server to just install the SSDT part without the vs_setup.exe and it fails straight away with 'Unknown error (0x80072efd) (0x80072EFD) and when I look in the log file it says failure to download from URL.

I tried to follow the instructions but with SSDT 2015 and that didn't work either.