Welcome to iMore! You have a couple of choices that I can think of:
1. Use an Apple Watch, which will display what your camera is seeing through the Camera app.
2. Purchase a lightning to HDMI adaptor. You'll be able to connect any HDMI-enabled monitor and it should mirror whatever is displayed in your iPhone. Here's one:
Lightning Digital AV Adapter for iPhone 5, 5C, 5S, 6, 6 Plus / iPad Retina Display, mini, mini 2, mini 3, Air, Air 2 / iPod touch 5th Generation, 6th Generation
https://www.amazon.com/dp/B009WHV3BM...mUvbUpU2694474