How do I edit the boundary of a terrain model that has been generated from a fieldbook

I have watched a dozen (or more videos) read several how-to's and I still cannot figure out how to edit the boundary of my terrain model. Would someone please walk me through this.

I am using OpenRoads Designer CONNECT Edition (Update 2). I have a large topo survey of a 5 mile long, winding road. Because the road winds so much, there are large areas of the terrain model that contain no data and need to be removed.

I have tried:

Adding a boundary using Terrain>Edit>Boundary Options>Add Boundary (This doesn't seem to do anything)

Adding a boundary using Terrain>Edit>Feature Mangement>Add Feature (This adds a the SmartLine I've drawn, processes, but the boundary and triangulations don't change)

Deactivating Survey Processing Rules on the fieldbook and deleting the extra triangles (This just doesn't work at all. Even when the rules are turned off, the terrain model won't allow editing)

I may have done any one of these things incorrectly, I don't know.

I have become very frustrated at what ought to be a simple process, and really need some help here.

Thanks