Can't play with my own subtitle when play MKV video

I have a video which has a matched *.ass file, they are named same(123.mkv and 123.ass),
but when I play this video in infuse on ATV4 or iPhone, it only let me select subtitle contained in MKV and I can’t select my own .ass file.
it also let me search subtitle at Opensubtitle website, but I need to use my own ass file,
am I doing something wrong?

In the infuse website, they say it supports ass.

What type of device are you streaming from?

Unfortunately external subtitles will not be available when streaming via UPnP or DLNA.