Using the Web App, I tested and trial many times and finally resolved this problem on my end.
This is the final file names which works:
forget about Plex suggestion, after the title file name and the year, one space, no hyphen, just the pt? or CD? or part? .
And then nothing after it, straight to the “.extention”.
But it raise a new question:
How to play Part 1 when the player is playing Part 2 or any later parts?