How to make a mob's limbs move normally?

I made a mob in Blockbench and his long arms are not moving correctly. That is, the upper part of the arms should be tied to the body, and the lower part should move, but for me the opposite is true: the upper part moves, and the lower part stands still. How can this be fixed? I understand that in Blockbench you need to use the "center of rotation tool" to bind the desired part of the body to something, but I put the center of rotation in many places and still the same result. Maybe you need some other tool?