Question: "Dear Plex-community, I installed the Plex-Server on my Sinology DS212j-NAS and a amazon Fire TV in order to watch my media on TV. Thing is, it will *only* play .mp4-files. When trying to play another format (i.e. .mkv) I get the following error-message: "It is not possible to play this video as no connection to the required Server could be found. I am far from being an IT-expert, but to me this error-message doesn't make sense as the connection to the server is definetly established. So I guess there is another problem with respect to the file-format... You may be my last resort of help! I hope that this pretty basic information is sufficient. Thanks in advance! "- from forums.plex.tv
Solution 1:Changing the following settings on the Amazon Fire TV Plex App maybe allow you to play mkv files.
Plex > Options > Settings > Device media profile > Media profile > Select "Force Direct Play".
Then uncheck all the "Device supports *" options.
However, the feedback unveils that this method get MKV files to play on Fire TV via Plex but no sound....(amazon fire stick). Because if you force it to Direct Play, and it is an MKV with AC3 audio, you won't get sound. You can direct play MKV's with AAC audio though. Honestly your best bet if you can't transcode, is to use an External player with the app, or as was previously suggested re-encode all your content to MP4 with AAC audio.
Solution 2:That error message could definitely be improved, but your NAS cannot transcode which it would need to do to play the formats/codecs the fire TV does not support natively via HLS streaming. Here is more info on NAS and which has a compatibility list of NASs that plex can be installed which includes details on which can transcode or not:
Solution 3:Simply convert your MKV files to MP4 with H.264 video and AAC audio with a MKV to MP4 converter so that you can play MKV on Amazon Fire TV with Plex.