Hi Alex,
I solved the error 891. It`s because of the download of the file attached in the MMS. You have to download it without any protocol (binary transfer - byte by byte). I made a short python script just for that and it works (if you want it, just ask). Now I do not get any errors. The MMS is send successfully but I never received it on the phone. I also contacted Sierra Wireless support and they confirmed that the commands are ok but the MMS isn’t received on the phone. Basically they have no answer why this happens. I’m looking forward on seeing what AT-commands you tried, maybe there is something we missed.