You can use your favourite telnet (not recommended) or ssh (recommended) application to connect to your Synology box and use it as a terminal.
- Enable the command line interface (CLI) from the Network Services
- Define the protocol and the user and make sure the user has password set
- Access the CLI
If you need more detailed instruction read https://www.synology.com/en-global/knowledgebase/DSM/help/DSM/AdminCenter/system_terminal