I don’t understand why this topic is considered solved. The indicated solution is not a solution, it’s a suggestion of how to go around the problem.
In the latest version, 5.5.4, this does not work. There seems to be no way to assign a custom mesh as the Simple Collision mesh. You can use a custom mesh as Complex only, which has very limited use. If you set Use Complex as Simple, instead of using the Custom Complex mesh, Unreal will use the Default Complex mesh. This is also not clearly explained in the UI. The custom mesh is indeed named Complex Collision Mesh, not Custom Collision Mesh. But nowhere does it say that the custom Complex Collision Mesh cannot be used in the Use Complex as Simple scenario.
Imo this limitation doesn’t make sense, we could have much better and more optimized custom collisions if we could use custom Complex as Simple collisions. Simple is limited to convex meshes that have to be mathematically accurately convex. And complex is the actual asset mesh, so too dense and not intended to be used as collision. Obviously, the suggested workaround is perfectly fine for Unreal, so you can use a custom Complex Collision mesh, as a separate mesh. So there’s no technical reason why we couldn’t have custom Simple Collision meshes. Because in this workaround, having to always keep track of 2 meshes for each mesh, 1 of them being invisible, seems to me like a complicated and impractical solution. So for any complicated asset that needs special attention, the practical solution still remains the good-old complicated setup of a bunch of simple convex meshes. Or going for the lazy optimized Complex Collision solution, which is in many cases impossible to use.