Okay so I have seen a lot of questions regarding similar issues but none are exactly the same so let me detail what I have done.
First off, I'm using Unity 5.6.4f1.
I have imported the characters, cross-platform input, and environment standard assets. I have added a quad to the scene to act as the ground and it has an active mesh collider. I can add a Rigidbody FPS Controller prefab and works perfectly fine. The WASD keys allow me to walk around and looking around with the mouse works as expected. I can also remove the RigidBody FPS Controller prefab, create a capsule object, add a RigidBody, add a capsule collider, add a camera, add the RigidBody FPS Controller script to the capsule, link the camera to the script, and everything still works fine.
Now here is my issue. If I remove capsule and children, add in a mesh (tried my own and some from store), add a rigidbody to the mesh, add a capsule collider to the mesh, add a camera to the mesh, add the rigidbody fps controller script to the mesh, and link the camera to the script, the WASD no longer work when I go to test the game. I can still look around with the mouse just fine, but I cannot move. Gravity is also still effecting the player if I put them in the air.
I have set the rigidbody settings to be the same as those on the rigidbody fps controller prefab, and I have added zero friction physics material to the mesh's capsule collider. I have no clue why it isn't working. Anybody have an idea?