You will need a computer with internet access to perform the initial download.
This is the most common issue with large offline installers. It means your 30GB file has a single bit flip. Fix: Use a torrent client to re-check the files (many legacy CryEngine builds are distributed via torrent legally for archival). Or, use 7-Zip to test the archive before running the installer. cryengine offline installer
One of the biggest nightmares in game development is a sudden engine update breaking your project. Online launchers often force updates or make it difficult to revert to an older version. An offline installer allows you to archive a specific version (e.g., CryEngine 5.6.8) forever. You can store this installer on a RAID drive, ensuring that your project is never broken by an automatic "improvement." You will need a computer with internet access