Yeah, the basic rules for collision are:
Each collision primitive needs to be a convex shape.
Each collision primitive must be named with a “UCX_” prefix on it that matches the main mesh.
So if you have a mesh called “MyCylinder” in your modeling application, each collision primitive needs to be named “UCX_MyCylinder” for Unreal to recognize it as collision.
The automatic tools inside of UnrealEd are good for some things but for something like a tunnel, I would just do it manually. It will be more accurate and you’ll have less issues.
Also, if your tunnel has 64 sides, you can probably get away with 16 collision primitives or maybe even 12. The lower you go, the rougher the collision gets but it’s expensive to have 64 collision primitives on a single mesh.