Visual Studio 2017 fails to install offline with “Unable to download installation files”

It worked!! All I had to do was install the certificates into the root CA! Something I missed noticing in the VS 2017 docs:

How to install from the offline installation folder

1. Install the certificates (They are in the “certificates” folder, which is in your Layout folder. )

2. Simply right-click each one and choose Install PFX.

3. Specify Local machine (not current user)
4. You can use an empty password
5. Run the installation file. For example, run:
   c:\vs2017offline\vs_enterprise.exe

— Microsoft Docs

Edit: Remember to install the certs using the Admin account on the PC, or it won’t work…