Is the magnet link one you created with Instant.io/WebTorrent, or is it off a torrent site? I'm wondering because of those tracker URLs I can see in the magnet URI.

If it's the latter, it's not going to work because the torrent needs to be seeded via WebTorrent(i.e. WebRTC). The desktop version of WebTorrent can download both regular torrents and web torrents because it's not limited to WebRTC for peer connections(whereas a browser is).