How to blur my background

Im trying to blur the background in my 2D game. I tried Depth of the field from post-precessing but,it doesn’t seem to work . It just blurs every object in the camera’s perspective. I want to Blur just the background. I tried different shaders but, nothing works.
note: I have multiple objects in the background

You might need to put things you wank to blur into RenderTexture and use another camera to catch the background.

And you might need to use layer mask to separate the background and foreground.

If your background is an image, you should rather blur it in an image editing program, becouse post processing only works on objects rendered by the camera