Unity AR how to put an object on a plane?

Question

Android

Good day!
There is a task to put a game object on the image from the camera.
The task is complicated by the fact that the object should not hang in the air, but should create the feeling that it is standing on the ground (table \ nightstand \ chair, etc.) and accordingly change its size depending on the distance of this surface.
Can anyone suggest a good library with which he himself worked to calculate the coordinates for an object based on an image?
Or examples where they did something similar?
The accuracy of the coordinates is not really very important - that is, flaws are permissible.
I myself have already rummaged through more than a dozen plugins / libs, I tried to adapt half of them, but the effect is not at all the same. No one has yet been able to put an object at absolutely any point, even if it's crooked. In addition to Tango, but there is also a special phone)
So far, openCV is in the lead, but there are few examples of working with it in the area I need in conjunction with the unit.
Either I don’t know English keywords well, the Russian community in Google turned out to be generally useless.
Why exactly coordinates - After studying so many libs, I realized that the best thing to do is to set the object in unity coordinates and rotate the unity camera based on data from the gyroscope and accelerometer. Solves a huge bunch of problems. But this is still IMHO which requires proof)

Answer 1

As a result, only SLAM suits me from the implemented functionality - which more or less meets my requirements)
Of the ready-made plugins, unfortunately only Kudan, Wikitude, MaxST and ios ARKit support it.
But the prices for them bite, except for arkit, but it is only on apples.
I will probably dig further openCV.

Answer 2

In Matlab there is an example of calculating the distance using the triangulation method (if you get a stereo image from two cameras).