How to create bootable flash drive from macOS High Sierra combo update
No, you cannot create a macOS USB Installer from the Combo Update, you need the full Install macOS High Sierra.app to do that.

Have a look at: How to create a bootable installer for macOS

If you cannot get it to download properly on your system using your connection, then you'll have to try from a different location and connection or have a friend download it, or go to an Apple Store.