Click Hidden
Lines Visible 
on the View toolbar.
Click Hidden
Lines Visible
on the View toolbar.
This shows the edges needed for the fillet.
Click Fillet
on the Features toolbar.
The radius is already set to 2mm to match the last fillet you added to the model.
Select four items for the fillet as shown:
The top face of the tall cylinder extrusion.
One edge on each side of the tall cylinder where it intersects the ring extrusion.
The edge of the hole that cuts through the tall cylinder on the bottom of the first extrusion.
Click 
.
Click Shaded
With Edges 
on the View toolbar.
