As you are talking about a MySQL-phpmyadmin on this NAS-System i expect it is more than "just a NAS" (NetworkAttachedStorage). It probably includes a webserver running php also.
So, if you are following a install-routine in your NAS Config UI to install phpmyadmin it is also creating aMysql-Database TOO. You need to check the documentation what mysql-credentials are used for this new Database.
Post a link to the documentation of your NAS-System. Maybe we can find some informations in there.