Unlocking x50 Windows Phones (like 550,950/XL) is much more harder than all the other series. If you feel confident, there are 2 ways to approach this.

WPInternals: https://www.wpinternals.net

vcREG: please take a look at this XDA thread

  1. upgrade the phone to Windows 10
  2. download and install vcREG and proceed to install the older NDTK version as instructed in the thread (VIDEO TUTORIAL, spanish only)
  3. don't do any CAP unlock, otherwise Interop Tools might not install properly
  4. use Interop Tools