Camera shutter button won’t work

On both my iPhone 7 Plus, and more so on my new iPhone XR, the camera shutter button only works sporadically. It is so frustrating to see that perfect shot and find that your shutter button won’t work. Sometimes I can face the camera down, the shutter button will then work (and I get a great pic of my feet), but when I bring the phone back up, the shutter button won’t work again. I have restarted my phone, done factory resets, updated my IOS, and every other suggestion Apple has given. Yet the problem persists. Please advise how this can be fixed. Thank you.