Try to download the media using this setup EXE program.
Put the iso on a folder on the computer C: system drive, mount the ISO and install from there. Alternatively, you can unzip the ISO file using 7zip or WinZip and the run SQL Server setup program from the unzipped content of the ISO file. Take in consideration that SQL Server setup program may fail when you try to install SQL Server on a compressed folder or drive. Read here for solution.