Yes, please remove the –link mariadb:libredb \ , as it is a convenience command to “link” hostname of “libredb” to another container called “mariadb” which was created in the tutorial
For your case, you should update -e DB_HOST=SYNO.IP \ and other SQL credentials to your MariaDB settings on Syno
-e DB_NAME=librenms \
-e DB_USER=librenms \
-e DB_PASS='libre_password' \
Please also update the permissions on the Syno MariaDB to allow access from the container