I actually had this issue as well. Fortunately, you can still install .NET 3.5 by using the .cab files inside of vanilla windows 10 installation media.
\sources\sxs\
This folder has .cab files for .NET 3.5, and by extracting the cab file and running the installers within, I got .NET 3.5 installed. You can also try pointing DISM to that sxs folder, but for me that did not work.