Don't know if this will apply to you or not, but will share why this happened to us and how we fixed it.
We have a live stream using Wowza. When we first set up our direct publisher channel, Roku would constantly say it was unreachable. For us, the reason was because it was calling for a live stream, but that URL wasn't streaming constantly. So, when Roku went to find it during a day we weren't streaming, it would think the link was broken.
So, what we've had to do is set up a constant stream (a looping video that says when our live stream happens) and then, when it's time to actually go live, our feed breaks into that live loop. When our live stream is over, it goes back to the looping video.
Once we did this, the unreachable video error went away. It seems that direct publisher requires that the HLS links point to something that is constantly playing.
Feel free to correct me if I'm wrong. This was our experience. And still today, if we turn off that constantly looping video when our live stream is not on, the roku feed says our HLS is unreachable and removes it from the channel until the next ping.