I recently upgraded from an iPhone 6s+ to iPhone XR purchased directly from Apple. (my service is through Telus, obvs )
For some strange reason, I can't seem to send a GIF as a MMS message. Had no issue with this on the 6s+.
The following all work:
-sending GIF via iMessage to another iPhone user.
-sending a photo via MMS to an Android user.
-sending a video via MMS to an Android user
However, if I pick a GIF, either one that I choose in the iPhone's #images search, or via Giphy that's integrated with the keyboard I use, and try to send it to an Android user (I've tried both with my wife, also a Telus subscriber, as well as to my sis-in-law, who uses Rogers), the message always comes up with "Not Delivered".
Chances are once had changed the SIM it was being sent as an RCS message vs. a standard MMS. RCS is designed to be more like an iMessage or WhatsApp message and will allow file transfers where a standard MMS would be limited to either a standard image or having to send a video clip.