Unable to enable .Net Framework 3.5, it shows the error code: 0x800F081F. As you can see the error above, it requires an internet connection but your internet connection is fine.
After some check, this computer is managed by Windows Update Server. Therefore, any traffic connecting to Microsoft will be managed by an update server.
Following is how to enable .NET Framework 3.5 in offline mode.
Mount the Windows Installation Disk to your Computer then Open PowerShell under Admin rights
Run this command:
DISM /Online /Enable-Feature /FeatureName:NetFx3 /Source:F:\sources\sxs /All /LimitAccess
Replace the F: letter with your actual drive.
Install in progress
Open Programs and Features
Verify your Computer has been enabled .NET Framework 3.5