Nokia devices have had impenetrable bootloaders for a while now, especially after the unofficial blackhat method was patched last month. HMD has finally remedied that, though, by launching its bootloader unlock program.
https://www.notebookcheck.net/Almost-two-years-after-launching-its-first-device-HMD-finally-provides-support-for-bootloader-unlocks.336500.0.html