1. Store your current config to XML file by Backup config menu
2. Open XML file by simple text editor
3. find tag <telnet>0</telnet> and replace 0 with 1
4. don’t touch anything else, otherwise DNS-300 don’t allow to restore config back
5. save XML file
6. restore config from XML file by Restore config menu
7. reboot DNS-300

Now you can use PuTTY to telnet to your DNS-300. Use your username and password to login. You can use command su and default root password to get root access.