Step 1 should be to click the orange Enable Remote Access button (from the page on your screenshot).
For many users, this will „just work“ with no extra steps. If it doesn’t, there’s a number of troubleshooting steps. Please keep in mind those can be a bit tricky, as there’s many different parties/elements involved when it comes to remote access (your Plex configuration, the NAS, your sisters router / home network setup and configuration, her Internet Service Provider…).
