I got my 3DS from someone else since 2017. I never bothered with the cameras until a couple of years ago, and I kept Googling to see if someone had an answer.
The behavior of my cameras is that they don't work on the 3DS camera app or the AR camera.
But this year, I finally installed homebrew on my 3DS and noticed that my front and back cameras work just fine when I use FBI to scan QR codes.
I even downloaded the DSi camera from the eShop, and it works.
I know I'm answering myself with a workaround for my problem, but I don't know why my 3DS camera app or any 3DS-related usage of the cameras doesn't work.
It doesn't work in the system settings to calibrate the cameras.
My only hypothesis is that one of the flex cables for the secondary camera doesn't work, but before repairing it, I'd like to know if anyone else has had the same issue.