It finally worked using ODK Briefcase!
Nothing else worked.
After setting Briefcase up, I pulled the submissions from the odk folder that I had found on the data collection tablets. It only worked when I pulled the whole odk folder, not just the instances folder. I deleted all the unwanted submissions from the instances folder first not to have to worry about duplication or overwriting something by mistake.
Then I pushed the submissions to the server using the url: https://kc.humanitarianresponse.info/
The submissions showed up instantly.
Thanks for the effort @Kal_Lam
