Actually,
Since I'm not giving up, I now can mount the drive using Samba on my Mac
smb://[router address]
But, each user I have created is refused. The only way to access it is by
logging in using the Router admin account. Then I access the entire attached drive.
But I have created two other usernames, whith limited access to one specific folder.
None of those users/passwords are accepted.
What I did: Setup > Storage > User Creation
The users are listed correctly with the good access rights.
Are those users and this page only for SharePort or universal to any kind of
access like SMB

Thanks for any clarification.