Download Mac Os High Sierra 10.13.6 Dmg

Before running the installer, disconnect from Wi-Fi (Ethernet too). Open Terminal and set the date back to when High Sierra was signed:

sudo date 0101000018

This sets the date to January 1st, 2018. Run the installer. Once finished, reconnect to the internet and re-sync your clock (System Preferences > Date & Time > Set automatically). Download Mac Os High Sierra 10.13.6 Dmg

This is the safest, fastest, and most reliable method. Apple maintains a hidden catalog of older installers accessible only via the command line. You do not need an existing macOS recovery partition to do this. This sets the date to January 1st, 2018

However, High Sierra no longer receives security updates (final patch: November 2020). Using it on internet-connected machines is not recommended for general users. This is the safest, fastest, and most reliable method