How to add navigation system on generated level?

Hello there, I’m planning on build a level to auto generate hex tiles and AI character can randomly move on it.
So I create a gamemode BP to spawn board BP at event beginplay, and board BP will generate hex tiles in construction script.

First part works fine.

Got some problems with last part, AI character did no move. I try to use NavMeshBoundsVolume as most tutorial did.
It worked if I put board BP in the level manually. I found a project on Epic Launcher did it without NavMeshBoundsVolume, but didn’t figure out how.

How can I add navigation after the hex tiles generated?