First let me caution you that your Roku 3 is now approaching eight years old, and it's always possible that something internal might fail.
Does the Roku still work with other channels/apps? We want to ensure your network connection is working correctly. Assuming it's working correctly with other apps, then we need to look at your PC. If this is a Windows PC, it's entirely possible that Microsoft issued an update the closed the computer firewall to permit access from other devices. They've done it before.
What are you using on your computer as your DLNA server? Is it the built in Windows Media service? You could check the Services app and make sure it's in the running state.
Plex should permit streaming any sort of media, so I'm very puzzled why it's not being recognized. Just be aware that the file extension (MP4, MKV, AVI, etc.) is simply a container type, and the media within the container can be in a variety of codecs. Since RMP used to play it, the file is likely in a supported container with supported codecs, or it's one of the few that Windows Media is able to transcode to a supported format. Windows Media is a terrible media server (in my opinion) but since it's included with Windows it does the (basic) job. But there are better (and free) options, with Plex being one. I personally use Serviio (disclaimer: I wrote the Roku profiles included with Serviio, but am not an employee), but Emby is also a good product.
Overall I suspect Windows has blocked your network from seeing the media server. Since RMP isn't even seeing your PC, but it does see Plex (which I assume is on the same PC), that points to a firewall block.
Dan
Roku Community Streaming Expert
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