Well, might take me a bit to get up to speed on what the whole situation is, but probably the best thing to do is break it down to individual steps, so you can see what’s happening.
If you are in the editor, you can turn on “Show Navigation” (press “P”) and you should be able to see the navigation mesh in the level. If you then use Play-in-Editor, and hit Eject (F8), then you should still be able to see the navmesh, as the game runs. As you place objects in the world, make sure that their collision component is set to Dynamic Obstacle. You should be able to witness the navmesh change as you do this. Path queries will work in the modified navmesh as you go.
There may be difficulties involved with pathing if you’re not using AIController or other built-in classes that may need support from AIController or Pawn or some such.
This might help some? Dynamic Navigation Mesh
This might too Unreal Engine 4 Tutorial - NavMesh Rebuild Realtime - YouTube