It’s not possible.
If the file is Direct Play’ing on your tablet, the font would have to be installed on the tablet. Which I doubt is possible.
Some video players are able to utilize fonts which have been added as an attachment into MKV files. However, this is deemed a high risk by many manufacturers and is therefore not enabled by them. You will usually only find this as a feature in open source players like Kodi or MPV.
If you want perfect formatting of ASS subs with all the appropriate fonts being used, you will either have to utilize one of the “big clients” (like Plex Desktop or Plex HTPC [which both use MPV]) or force the subtitles to get “burned in” to the video picture by the server (which means transcoding and therefore requires a potent server machine, which a Synology is not).