Thanks to my son-in-law, I am now enjoying the benefits of using Plex on our Roku. He casually made the suggestion, when I was grumbling about not being able to play back some local content on the Roku.
It was a real "Duh!" moment for me. I have an HTPC in the basement, hooked up to a TV and running KODI. However, sometimes I just want to watch stuff upstairs. On those occasions I would either play contents from a USB stick, or from the HTPC via DLNA on Windows 10.
However, some content would not play properly on the native Roku player, depending on the video format or the way the audio was encoded. "Oh," said Jacob, "why don't you just use Plex?".
As I said, duh!
It took all of 10 minutes to download and install Plex server on the Windows 10 HTPC box, and add the free Plex app to the Roku. Unlike the Android app, the Plex app for Roku gives quite good functionality with the free version (the free Android app is severely limited).
Now I can easily stream local content to the Roku, which is all I was trying to do in the first place.