Note that many hosts have their servers configured to silently ignore permission changes through FTP. Double check that you actually are changing permissions. If not, use your host's control panel > file manager to change them.
Always start with 755. If SMF/Aeva tells you it can't write to the folder, try 775. As a last resort, 777 (and see if you can change back to 755 when you're done uploading). If none of that works, you'll have to ask your host for suggestions.