How to add external sub to play with Mounted ISO files

Can someone please show me how i can use external sub to play with mounted ISO files. I did put them in the same folder and replace the same name both ISO and SRT files but subtitles does not showing when playback
Thank you
p/s when playback i can choose to download subtitles directly from opensubtitles and it works well. Unfortunately no my language from this site

What type of device are you streaming from? Are you using UPnP or DLNA to connect, by chance?