Technically this can not be illigal( hopefully), but I read from DSOG that following these steps you can enable ranger mode on pc:
1: open regedit
2: Navigate to HKEY_CURRENT_USER/Software/Valve/Steam/Apps/43160
3: Create a new String value called ListOfInstalledDLC with Value data 222801
4: Open game New game Select Hard Ranger Mode
If you cannot find a folder named 43160 , simply browse until you find the folder that contains Metro: Last Light, and do the aforementioned changes.
Have not tried this myself though!