Hmm. Ok so your CID is HTC__621. Here is what you should do. Download this firmware file for Oreo (this is only firmware without complete Android OS but it will do the job since you wanna install custom ROM after this). It is 100MB and it will work on S-on device with SD card method (explained below) hopefully:
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Once you have downloaded, rename the file exactly as 2PS6IMG.zip. Copy this file to SD card (don't put inside any folder). Put the SD card to phone. Boot the phone to download mode. It will automatically start flashing. Let me know how it goes.