7-Zip is a popular, free, open-source alternative that handles multi-part archives.

Do you have of this pack in one folder, or is a specific piece missing? How to merge .part files using WinRar

The software will automatically pull data from part2.rar and any subsequent parts to rebuild the original file(s). :

: This happens if a part is missing or renamed incorrectly. Ensure all files follow the exact same naming convention except for the part number.