Short version: If you can’t get files off your DJI Action 2 (or Osmo Action 3 or 4...or Pocket 3) via the DJI Mimo app on your iPhone (or other iOS device), temporarily turn off cellular data.
The video also has a big shout out to the awesome DJI support team, especially Wei.
P.S. This video was created for my Action 2, but it works as a needed solution for my Osmo Action 3 and 4 too, so I updated the title and description to reflect that (even though the video won't).