Mesh collider falling through floor. Cannot troubleshoot!

Hey guys.

I'm setting up a new basic scene and testing some things out. I've imported my first character models and got animations working. I used a basic cube for a floor and my character's colliders seem to work great.

However, my imported "Golem" model (free asset from store) falls through the floor whenever I try using a mesh collider. Rigid body works fine when using a basic primitive shape like a box or capsule collider.

Both the floor and the Golem model have the Rigid bodies applied. A mesh collider has only been applied to the Golem model on the highest most parent in hierarchy. I've tried both convex and normal map on all the child's (Bone structure) and that doesn't work either. I have no idea what to do at this point..

Any help goes much appreciated. Thank you!