Install the .NET 4.8 version on our Windows Core Server.

Run this to find out the version number of the currently installed version of .NET.
reg query "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\NET Framework Setup\NDP\v4\Full" /v version

To install .NET on your Windows Core Server.
Change the Directory to the location where you have the .NET file stored on your server.
C:\Users\YourUser> G: G:\>cd apps
Get a list of files in the Directory.
G:\apps>dir
Switch over to Powershell.
G:\apps>Powershell
Now, load the DOTNET file and install it.
PS G:\>./DOTNET-4.8.exe /q
Once the installation is complete, the system will reboot, and then you can run the code above to find the version number of your newly installed .NET Framework.