More in general If you have a fully calibrated camera you can project the 4 bounding box from the screen coordinates back into the 3d world but you need to know the Z of each bounding box point that currently it is not estimated by your object detection model.
You could try also to use a stereo camera setup or there are approaches to estimate the monocular depth but you need to check if they could have enought accurancy for your specific use case:

  
      

      arXiv.org
  

  
    

Single Image Depth Estimation: An Overview 4

  We review solutions to the problem of depth estimation, arguably the most
important subtask in scene understanding. We focus on the single image depth
estimation problem. Due to its properties, the single image depth estimation
problem is currently...