By default when playback starts their will be an overlay box that contains the play/pause button etc. This overlay shows for around 5s before disappearing. Using the remote control arrows, navigate to the bottom right of the screen where the subtitles text reads SUBTITLES OFF. Once highlighted in blue press the select button in the centre of the arrow keypad and the subtitle text will then read SUBTITLES ON. The subtitles should display a few seconds later. Vice versa to switch subtitles off.
Note that you may not be able to select subtitles setting until the content begins. You may also need to press any arrow or the select key to bring up the playback controls if they have disappeared during playback.
*Same method for Fire TV