Answer
Navigate to the custom page itself and copy the client name and page name directly out of its URL, which follows the pattern /web/page/[client name]/[page name] — then set the Navigator Designer entry's Client and Page fields to exactly those values. A Navigator entry that doesn't match the custom page's actual generated client and page names (for example, from guessing or typing similar-looking names) won't resolve correctly, which is what makes the entry appear broken or non-functional even though the underlying custom page itself is configured correctly.