Solutions:

1. Ctrl+Atl+t
2. Type nmcli con edit type pppoe con-name ANY_NAME_OF_DSL_YOU_LIKE, which will open an interactive prompt.
3. Type set pppoe.username YOUR_DSL_ACCOUNT, and so as the password.
4. Type save, enter.
5. Type quit, enter.
6. Finally, you've got this:

Have a nice day in Ubuntu 18.04~