how do you assign a root for ragdoll

Use the actual root game object as the root. Not the armature or root bone -- the actual top object. Give it a rigidbody and collider as normal, but, for the collider, pick the "main" bone, like the spine, and shape the collider to it.

Then every other bone works as normal -- give it a collider, RB and hinge and hook it to the appropriate bone. But, for bones connected to the root bone, hook them instead to the real root, from step 1. The actual main spine bone is completely unused.

This way, the whole object has a collider and RB, and moves as normal. The collider seems too small, but all the kid colliders count as yours, so the real collider is a "compound collider" using all the bone colliders.