UPDATE: As of November 12, in-game mission downloader works again. No tweaks/hacks required.   The game has an option to store FMs on mirrors. I'm pretty sure all of them are located on mirrors. However, the game itself has no way to learn about new missions and their locations, except for ask someone else. That's why when you click "Download Missions", TDM downloads an XML file from one of the URLs stored in cvar tdm_mission_list_urls, which serves as a catalogue of all FMs available.