You still have an out! You’ll need a USB drive of some sort. With it plugged in (leaving OWC out).
Then install High Sierra onto the external as if its the internal once the OS is installed the firmware will be also updated. Then download the OS installer from here [https://support.apple.com/en-us/HT208969|How to upgrade to macOS High Sierra]. Jump down to Step 4 to get to the blue URL link “Get macOS High Sierra” so you have it on your bootable drive to then install it first onto the OWC drive, then if you want to upgrade do it then.
Reference: [https://support.apple.com/en-us/HT201255|Mac startup key combinations]