HTC One Mini 2 - Demo unit, how can I unlock it so it can become normal?

[AC Question]

HTC One Mini 2 - Demo unit, unlock it so it become normal.

I work in a store and I've got a HTC One Mini 2 since we dont sell it anymore. It supports GSM and works perfectly, but the problem is it has a demo on it.

I cant enable developer settings aswell as other software settings. Is there any way to flash a custom rom on the device without having developer settings available?

Thanks for any help!

PS: European model