Is using the box safe?

Hi, my HTC one M8 eye isn't working for about one week now because I installed a wrong ROM on it. I tried many methods to get it back working but all have failed. I messaged HTC service center in my country and they said they can't support it even if I pay, since it was damaged in a non technical way. So I sent it to a guy in a Mobile repair shop and after two days he said, something is preventing any flash ROM from being installed and that he has to use the box method to remove the cid and make the phone accept any custom ROM. Now, I don't understand what was he talking about and I fear this would lead to more damage. What shall I do?

Thank you in advance .

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