A public NAS..? So literally everyone on this planet can access your NAS device? You are a very generous man, but with a very insecure network setup.
Maybe I should consider getting a password
Indexing is something else than accessing. If you can access the files by Kodi, then the network connection itself is working. After that, you may have to double-check which user/file rights are active. Read-only, wrong user.. something must be off.
I can access the files when I use KODI v18.0-RC3 on my Windows 7 PC and on a RaspberryPi with OpenElec V7.0.1 (KODI v16.1) so i don't think its my NAS that is the problem.