Nut sure if it will work, but have you renamed the sub files same as the video file (except the extension of course) and placed in the same folder as the video?
KODI is an completely independent "product" from HiMedia, so there are no cross-elements regarding playback or anything else, it's just the Android version of a generic Media Player, but on most Android media boxes it still serves just as Media Browser, juts like on HiMedia, because there are too much variables to modify that can't be done for each hardware equally, so it's playback options stuck at far from perfect. (Tho' have much better compatibility with PC, Win/Linux and Libre/Core Elec).
Some brands, like Zidoo or Minix have their own, specially modified version of KODI (ZDMC/XBMC) to gain most of the hardware features their Boxes capable of, but that's another story and won't happen in HiMedia front anytime soon...
Tip: those subtitle setting may have been hidden a bit in the Archos Player's menu, you may have to dig a bit more deeply...