Tesselating navmesh

I'm sorry for crossposting this but I realise this should be been here I have a flat terrain which when baked is being tesselated by some unknown means into approximately 20x20 grid. since there's no geometric detail to carve up the terrain, the fidelity of the pathfinding is limited to the 20x20 verts on the edges. I have worked around this, but for fear of that introducing bugs and having no use for holes in this navmesh, I was wondering if anyone knows of any way to increase the tesselation density or add verts to the mesh in some way?

I downloaded Unity's extended navmesh stuff from git-hub which had some promising claims but failed in practice.