First of all, It would be great if you can share a video of the issue, showing the guards not moving.
Is it possible that they are actually moving but you cannot see the navmesh near them? Remember that the navmesh is only shown in a small area around the player but, regardless, it is active even when not visible. Maybe it is being generated and you cannot see it because you are not close enough to that area.
One extra thing you can test is to use “AI Patrol Path Node” devices to create paths for the guards and check if they use them even if navmesh is not showing.