Till now, there is no official iOS app to add/manage torrent tasks of TransmissionBT. The only working application is iControlbits which is available to Jailbroken iOS users.
So there is not much choice left for non-jailbroken users to manage Tranmission tasks. We can use Safari web browser to view and delete the downloading task but no way to add torrent file as iOS has a very strictly privilege control on file dealing. It is impossible to download a torrent file and upload it via Safari browser.
After lot of research, I finally worked out a solution.
Just keep it simple.
1. leverage Resilio sync (formerly BittorrentSync).
a. install Resilio sync app on iOS
b. install Resilio sync client on the server running Transmission service
c. create sharefolder on the server with Read/Writer access
d. link Resilio iOS app and server client
2. enable transmission watch folder function
a. enable transmission watch-folder function on settings.json configuration
b. point the watch folder to that sharefolder sync with Resilio iOS
3. download the torrent file using iOS safari browser then choose ‘open with more’ , Resilio Sync add file to the folder
4. once the downloaded torrent file is automatically synced to remote Transmission server, Transmission will pick it up and start downloading
5. manage the downloading task via Transmission WebGUI though.