Hi Nikita3D,
is this what your after.
The problem is where the rib joins the hex and the top dia.
The flat on the hex is same width as the rib so you have to many edges converging.
I created a curve that gave a wider flat on the hex and also trimmed the rib and top dia.
This curve was revolved (caps off) and isect boolean .
This gave less converging edges and the fillet tool worked.
Hope thats clear.
Cheers
Barry