Microsoft has discontinued the service that was required for dev-unlocking WP devices. Windows 10 Mobile can still be dev-unlocked, because that's built into the OS, but the HTC 8X never got an official W10M build.
Your options, as I see them:
- Get a newer phone. The 8X is something like 7 years old now, in smartphone terms that's well past obsolete. You can still use Windows Phone apps on a device running W10M, like the HTC One M8, Lumia 950, or so on.
- Try to find an unofficial ROM for the 8X that either includes W10M or is already dev-unlocked, and flash it using the official flashing tools (WDRT installs command-line tools such as
thor2 that you can use to flash ROMs).
- Try to use WPInternals to get USB Mass Storage access to the phone's internal storage, at which point you can edit its registry directly to enable dev unlock (and other changes).
All other approaches I can think of for the 8X would require already being dev-unlocked. I never had one myself, and some people who did managed other unlocks for the 8X including "interop unlock" which is a super-dev-unlock that requires editing the registry directly (usually dev-unlock was an intermediate step to get that, though, and back then you could still use the Microsoft developer registration tool). I'm afraid at this point any WP device that can't run W10M is basically worthless, capable only of running a very outdated browser and email client and unable to install almost anything. W10M isn't supported, per se, anymore either, but at least its store still works (last I checked) and its browser is outdated but not nearly so badly.
Thus, if you can't manage to get W10M on your 8X - and bear in mind that while it meets the minimum requirements, it won't be able to run W10M well - it's probably time to retire it.