How to make external subtitles appear automatically?

Good morning all,
I'm new to the Plex world and all in all it's a great product but I do have 1 problem that I can't seem to get to fix.
I use external SRT subtitles but every time that I want to watch a movie\series, I have to choose the subtitle stream by hand, I have read that I need to change the name of the subtitle name to reflect the 3 leter code, and it worked, the server identifies it as the language, however, I still have to select them every time I go in to watch.
My server is installed on a Windows 10 machine and I use a Roku LT device to connect to it.
Is there a way to make the server select the external SRT file automatically?
Many thanks,
Ice