DirectX 12 is natively integrated into Windows 10 and does not have a standalone official offline installer . Updates for DirectX 12 are exclusively delivered through Windows Update
If you have an internet connection, this small tool identifies and downloads only the specific files your system is missing. Download: DirectX End-User Runtime Web Installer How to Install DirectX 12 Offline on Windows 10 directx 12 windows 10 64 bit offline installer link
🛠️ How to Update/Install DirectX 12 on Windows 10 (64-bit) DirectX 12 is natively integrated into Windows 10
Even with the correct offline installer, you might encounter issues. Here is how to fix them. directx 12 windows 10 64 bit offline installer link