At this point let's try a few things.
Firstly, I think it makes sense to see what the log says, you can find the button "export debug report" in the settings
Secondly, if you export your profile on Android do resources indeed there (and you can open them) or they're, indeed fail to be downloaded in the first place?
It might be helpful to see if Joplin desktop on other operating system would sync those photos.
Please check that you're running latest web view. It's not a requirement but in my experience it helps with performance and stability of the app.
Also, maybe you can copy your data to another cloud provider so we can cut off the cloud interferring with the sync.
I wonder what's the format of those photos?
Do all attachments fail to sync or only photos?
Are they unusually large?
Do you have enough space on the phone to keep them?
