Thanks Mediadisconnect!
I changed to HLS link, it Works finally. Now I just wait for Roku approve for my test video.
In the mean time, If this works, I have more videos to put on my channel, do you know
1. shall I put all URL on the same .json file?
2. Can I still use myjson.com as a hosting site for my file, or I need to upload the file to my own website through hostgator - then I need to figure out how?