I always suggest trying myjson.com and pasting your feed in there. If that works, then you'll know its related to the way the json file is hosted or the url path etc
Not necessarily. When you use myjson.com you're not uploading a file, but copying text from the clipboard into a web form, which myjson.com uses to create its own file with its own encoding/line endings, etc.
When you upload an actual file from your computer to a web host, then that's the file that will be hosted. If that file is not compatible with what Direct Publisher requires, the file may not work when hosted on your web server, but may appear to work when using myjson.com. DP seems to require UTF-8 encoding without
a Byte Order Mark (BOM). If you try to upload a file that uses a different encoding, such as UTF-16, or a file that contains a UTF-8 BOM, then DP will reject it. Even though mysjon.com may accept the file, the real problem is with the file itself and the way it's encoded, and not with the web host.
For an example where this happened, see this thread: https://forums.roku.com/viewtopic.php?f=100&t=100756