A quick search of your model number turned up the manual, which sent me to go.roku.com/rokumediaplayer for a list of supported formats. DTS is listed, but it also specifically says:

Note: DTS-HD and lossless DTS are not supported.

So you’ll have to remux the MKV file with a standard DTS encode of the mix of your choice, or a Dolby Digital/AC-3 encode.

However, FLAC and PCM (.wav) audio are supported, so if you still want lossless you can remux your MKV with an uncompressed PCM encode or a lossless FLAC encode. I don’t have the mono mix in any of these flavors, but I’m sure someone can help you out with that.