This page here https://docs.fileformat.com/audio/m3u/ gives you a pretty good example of configuring .m3u file formats. (ignore the fact that it is for audio m3u, just look at structure).
The example provided by @atc98092 in the link you posted https://community.roku.com/t5/Solving-playback-issues/Roku-Media-Player-playlists-will-not-play-my-M... is a good example to configure the playlist m3u file.
As another example regarding directories, see if this makes sense.
Assume you have USB drive with playlist.m3u file and two folders (foo1 and foo2). In foo1 you have movie1.mp4 and movie2.mp4. In foo2, you have movie3.mp4 and movie4.mp4.
Your playlist.m3u would be configured as follows.
#EXTINF:,This is Movie1.mp4
#EXTINF:,This is Movie2.mp4
#EXTINF:,This is Movie3.mp4
#EXTINF:,This is Movie4.mp4
In the above format, the "This is Movie1" will be displayed as the title in your playlist for movie1.mp4. (Note the comma (,) before the Title you want displayed. Normally the track length time is placed before the comma if you want that displayed in info. The line below that is the path and actual name of your file.
If you use a media player like VLC to build the playlist.m3u file the EXTINFO will be created automatically for you (timetrack and title).
Also, if using media players to build the m3u files, make sure to use the exact name of your mp4 file. (VLC may add %20 to spaces in names of titles so you will want to remove those before playing in Roku Media Player (RMP). RMP does not seem to recognize the %20 as a space and your file will not be found. RMP will recognize spaces so just type or copy/paste exact name of your mp4 into this field.
Hopefully the above makes sense to you. If still having issues, post back with your actual directory structure with some sample titles.
Roku Community Streaming Expert
Just another Roku user... I am not a Roku employee.
Insignia RokuTV, Ultra 4660x, Premiere+ 3921, Express 4k+ 3941