How can I fix permissions with Wordpress file uploads?

Hello, I had previously set up my server with this guide: https://www.digitalocean.com/community/tutorials/how-to-configure-secure-updates-and-installations-in-wordpress-on-ubuntu

Basically, I set wordpress to use the wp-user, and set the wp-user to own the entire html directory. Another thing I did was to add my main user to the wp-user group, so I could edit files in the html directory on my FTP client.

Now, I have another wordpress installation running on a subdirectory of the html directory. I told it to use the same user and authentication as the first installation – but it didn’t work. Theme installation would be fine, for some reason, but media uploads are impossible. It just says “could not create directory …”

After messing with it quite a lot, I’ve managed to screw up media uploads on my first installation. Again, theme installs work fine. But media uploads have a different error on the first installation: ‘The uploaded file could not be moved…’

All files and folders in the html directory are owned by wp-user, and have a group of wp-user. Folders all have a permission of drwxr-xr-x, and files all have a permission of -rw-r–r–.

No idea what’s happening here. Any ideas?