newbie question. No "oem" toggle unlock in dev options to flash phone

I just purchased a SM-G986usqu1atct, service provider sw, sm-G986u_oyn_xaa_qq_007 XAA/XAA/ATT. I could not get it to make/receive calls now do any software updates. Sent to Samsung repair facility and was told the phone is a "demo" unit and cannot be repaired? Tried to return the phone but they wanted a 50% restocking fee. Nightmare.

So, I have been browsing these forums and have tried unsucessfully to re-flash it. I was able to get one of the firmware's to install but when rebooted it...

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