To enable English subtitles, click the gear icon on the bottom right side, click "Captions", and then "EN".
On desktop browsers you should have no issues viewing subtitles.
On iOS devices, the video has to be minimized (not full screen) in order for subtitles to show. This is sadly the case in all the video players on iOS Safari (even Youtube does not show subtitles when played in a browser, only in the native app) โ there is currently no technical workaround.
After you enter the full screen, just pinch the video and you will exit it - this should show the subtitles fine . See the reference screenshot on iPhone below.