Is there an album "Unable to upload" on your iPhone?
Do you remember, when this problem first occured?
Try to view all items enlarged you saved to Photos since the problem first occurred. Also try to add adjustments.
Typically the problematic item may be a file you saved from the web or hast been shared with you, or edited in an older external editor. older videos, or downloads from the web, that resulted in a link instead of a file, photo editing extensions that turn videos into Live Photos may create incompatible files.
You wrote, you have no Mac any longer, but do you have access to any computer, where you can open your iCloud webpage, so you can compare the recent items on your iPhone to the items in iCloud? Try to open your iCloud webpage at www.icloud.com on a computer (you need the desktop version of the website, not the mobile version). Perhaps you can see this way, which file is missing in iCloud.