I had a coworker who struggled to make a modal in a secondary router-outlet appear.

The Angular routing problem he bumped into acted as if the route for the secondary router-outlet could not be found. This turned out to be due to the fact that it was a child of a route that had an empty string for a path. It turns out you can't do that. You primary route must have a segment of the route as a path in order for the secondary router-outlet stuff to behave itself.