DyanamicRouter.cs

public class DynamicRouter : IRouter
{
  private IRouter _router;
  
  public DynamicRouter(IRouter router)
  {
    _router = router;
  }

  public Task RouteAsync(RouteContext context)
    => _router.RouteAsync(context);
    
  public VirtualPathData GetVirtualPathData(VirtualPathContext context)
    => _router.GetVirtualPathData(context);
    
  public void SetRouter(IRouter router)
  {
    _router = router;
  }
}

var router = new DynamicRouter(routes);
app.UseRouter(router);