Hi Feng,
There are no backdoor logins or other ways of accessing the device if you've lost the root password. If credentials to access the Digi CM16 are unknown, the only way to regain access is to Factory Default the Digi CM16, at which point you'll be able to access it using the Factory Default username/password combo.
The process to factory default your product is found on page 148 here:
https://www.digi.com/resources/documentation/digidocs/pdfs/90000301-88.pdf
The factory default username/password are listed in the Quick Start Guide located here:
https://www.digi.com/resources/documentation/DigiDocs/pdfs/90000300-88.pdf