Unlocking x50 Windows Phones (like 550,950/XL) is much more harder than all the other series, that's why we don't officially state compatibility with such phones. Anyway, if you're able to unlock your x50, WinWazzapMigrator will work flawlessly with the extracted messages.db. If you feel confident, there are 3 ways to approach this.

1) Port your WhatsApp to a supported Lumia phone (suggested: Lumia 640XL) then unlock and extract from there - you can port the backup to another Lumia through embeeded WhatsApp backup feature (SD or OneDrive)

2) WPInternals (suggested): step-by-step tutorial

3) 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