From the Main Menu follow the path of:
Multiplayer --> Browse --> Browse networks.
Join any network on the right of the screen that is public and allows any member. Alternatively, you can search for TrueAchievements (call sign: TA) and join that network instead. The achievement should pop moments later.
If you can only see the Advocate Network when you try browsing, chances are this is because of something to do with your Xbox Live privacy settings.
To rectify this, click the Guide button on your controller and scroll to the bottom of the menu to where it says 'All Settings'. From there go to:
Account --> Privacy & Online.
Make sure your settings are turned to 'Adult Defaults'. Quit and restart Titanfall 2 and the option to browse networks should now be there :)