//EDIT: I made the assumption you wanted to create a mesh. If you want a curve solution, you'll find it at the end.
//2. EDIT: Sorry I'm an idiot I somehow thought you wrote circular profile with a diameter of 4 mm... I'll correct the values for the 2 mm version, but the images are still for a diameter of 4 mm.
Mesh version: A hoop with a circular profile? So you mean what is usually called a torus? What about just adding a torus with Shift+A > Mesh > Torus?
Then set Major Radius to 20 mm and Minor Radius to 1 mm in Dimension Mode > Major/Minor or maybe Exterior Radius to 20 mm and Interior Radius to 18 mm in Exterior/Interior mode?
Whereas the Major/Minor example I gave here actually creates an overall diameter of 42 mm instead of 40 mm, I don't know if you want 40 mm overall dimensions or 40 mm from one center of the circular profile to the opposite profile's center.
About the bevel dimensions: I guess you have scaled the object in Object Mode and didn't apply the scale: Ctrl+A > Apply > Scale.
Curve version: If you're instead going to do this with curves, add a curve circle with Shift+A > Curve > Circle. Set the Radius to 20 mm and under Object Data Properties > Geometry > Bevel set a Depth of 1 mm. This also results in an overall width of 42 mm because the radius goes from one profile center to the opposite. If you want an overall width of 40 mm, set the Radius to 19 mm.
Excursion: Comparing Dimensions Mode options of the Torus, Major/Minor and Exterior/Interior.
In both modes I created a torus object which is overall 10 m wide in diameter and has a circular ring profile with a diameter of 2 m.