what version is on there now? Maybe you can plug it into a windows PC, use Windows device recovery tool and update the version that is available on that. It would wipe everything out and put fresh software though.
Something doesn't seem right. It almost sounds like my old laptop, x200t... on anniversary update, it was acting weird and updates very slow or not at all. I gave it to my parents a while ago so it wasn't getting a lot of attention.
This past weekend, I took the laptop back and was able to download the new version manually and update the computer.
Now for a phone, you can't do this but it's worth considering to reset the phone or use the windows device recovery tool.
Sorry, it's too hard to troubleshoot to fix they symptom. Easier to just reset the software