Did you try the following Recovery key combination as in your case Shift-Option-command-R mayb ehlep

1. Command - R: Install the latest macOS that was installed on your Mac.

2. Option - Command - R: Upgrade to the latest macOS that is compatible with your Mac.

3. Shift - Option - Command - R: Install the macOS that came with your Mac, or the closest version still available.

See if your High Serria is still available from it. I think it should be as you can still get it from App Store.

Did you try the following Recovery key combination as in your case Shift-Option-command-R may help

1. Command - R: Install the latest macOS that was installed on your Mac.

2. Option - Command - R: Upgrade to the latest macOS that is compatible with your Mac.

3. Shift - Option - Command - R: Install the macOS that came with your Mac, or the closest version still available.

See if your High Serria is still available from it. I think it should be as you can still get it from App Store.