Via help@inat: an AT&T user was able to get photos loading again by using the AT&T Smart Home Manager app and allowing access to https://inaturalist-open-data.s3.amazonaws.com/
For some reason AT&T doesn’t like it. We’ve run it through multiple security checks and it’s passed them all, and the SSL security certificate for it is valid. So I suspect it’s a mistake on AT&T’s end.