"When installing SSDT manually it downloads a shell copy of Visual Studio 2017 from the Internet. However, if I get the offline copy of SSDT for VS2017 it does not include the Shell copy of VS2017 so the install fails because no Visual Studio 2017 instance is present. I need this for an internal network installation either via SCCM or wide unattended distribution."
This request makes sense for a lot of large enterprises where internet access may be restricted for security considerations.
Steps to accomplish this:
The overall approach is to download the required files on a computer that does have internet access. Then later use those bits on an internal network to install on multiple computers that do not have internet access.