Stremio doesn’t need a play with option really. The vid player is great on it’s own. Subtitles are easily configured in the settings and by having the subtitle add on enabled. No downloading.
I guess I don’t understand what it is you’re trying to do. You don’t “download” subtitles in Stremio, if the movie or series has a subtitle package and you have open subtitles addon installed and subtitles configured in the settings, then playing that movie etc. subs will be displayed, no “downloading” that I’m aware of. Ahhh I forgot one thing, are you attempting to load a “custom” subtitle .srt file?
You have extensive control over the different aspects of the subtitles. To change them, click on the subtitles icon at the top right toolbar while watching a video. Here are the subtitles settings:
Change between embedded and external subtitles
Choose the subtitles language
Choose the subtitles source. For each language, there will be at least one subtitles source
Increase or decrease the subtitles delay
Increase or decrease the size of the subtitles
Increase or decrease the subtitles vertical offset
Use black background color for subtitles
If more than one source you can try changing it.
Now it is a tad difficult to get to if you don’t know how. When the movie starts click the “pause/play” Then the large “play” arrow should be highlighted, click up, then scroll right to highlight the subtitle icon, second from the right, just left of the hamburger, then you can turn them on or off here. You can also configure sub languages etc. in the settings menu. now in addons go to the top right and click on community and change it to “official” and scroll down to make sure “OpenSubtitles” is installed. Now in addons you also have other subtitle addons you can install. I think there is a way to add more sub sources but I have to take a look, it’s been a long time since I’ve dug into sub settings in Stremio. If I find out how I’ll get back to you. http://subtitlesv2.strem.io/
If you go to addons then at the top click the search and type in titlovi or wizdom then enter, those two subtitle addons will show and you can add them. That will give you 3 sub sources.