It's frustrating when your remote isn't working. However, sometimes you can't see the light transmitted to tell if it's the batteries. Business Insider says hat's because it's infrared, which our eyes cannot see, but your phone can. Well, most phones can.
Go to the camera on your phone and point the remote at it, then start pressing buttons.
When you start pushing the buttons you can see it through you phone's camera. If that light isn't working, you probably need new batteries.
Why can your phone see this?
Your cell phone camera is more sensitive to light than the human eyes. To make it work on iPhones, use the selfie camera. On Andriods just use the regular camera.