In the add network location for the path enter just the ip itself for the server name and nothing else on that line (the smb:// gets added by the protocol option above it) and you will likely have the best luck with using credentials. You may also need to go into settings>services and set the SMB version to min/max v1.
It may also be possible that there is some incorrect credential info that got set along the way that may be causing issues. I would ssh into your device and take a look in ~/.kodi/userdata/passwords.xml. You should see where and how it is adding the credentials to the network path so you can judge if their is an error.