MMS not sending through Textra (on Project Fi)

I've not been able to send an MMS (GIF, photo, etc.) using Textra on Project Fi. The native messaging app on my 6P has absolutely no issues. I've tried the workarounds suggested within Textra - switching to Legacy, imposing a message size limit - to no avail. No matter what I get those 3 green dots suggesting the app is attempting to send.

Has anyone else experienced this issue and found a way to fix it? Thank you!