If a direct offline installer isn't available, you can still prepare for an offline installation:
Go to:
C:\ProgramData\Epic\EpicOnlineServices\
Or
[YourInstallDrive]\Epic Games\UE_4.27\
Inside, you will see a folder named ManifestCache or a series of .chunk files. Copy this entire folder to an external hard drive or USB stick. Unreal Engine 4 Download Offline Installer
Pro Tip: The total size is roughly 25GB to 45GB depending on the version (Android/iOS content adds bulk).
If Epic Launcher insists on logging in when you have no internet: If a direct offline installer isn't available, you
Note: This is the most reliable method for the average user. It uses the Launcher to create an installable package.
If you have the Epic Games Launcher installed on a primary PC with internet access, you can essentially create your own offline installer setup. Pro Tip: The total size is roughly 25GB
Pros: Guaranteed to be the correct version. Cons: Requires one initial internet connection and the Launcher installed once.