Use the default credentials to log in the first time:

• Username: cumulus
• Password: cumulus

$ ssh cumulus@<ipaddr>
Warning: Permanently added '<ipaddr>' (ECDSA) to the list of known hosts.
Ubuntu 18.04.5 LTS
cumulus@<ipaddr>'s password:
You are required to change your password immediately (root enforced)
System information as of Thu Dec  3 21:35:42 UTC 2020
System load:  0.09              Processes:           120
Usage of /:   8.1% of 61.86GB   Users logged in:     0
Memory usage: 5%                IP address for eth0: <ipaddr>
Swap usage:   0%
WARNING: Your password has expired.
You must change your password now and login again!
Changing password for cumulus.
(current) UNIX password: cumulus
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully
Connection to <ipaddr> closed.

Log in again with your new password.

$ ssh cumulus@<ipaddr>
Warning: Permanently added '<ipaddr>' (ECDSA) to the list of known hosts.
Ubuntu 18.04.5 LTS
cumulus@<ipaddr>'s password:
  System information as of Thu Dec  3 21:35:59 UTC 2020
  System load:  0.07              Processes:           121
  Usage of /:   8.1% of 61.86GB   Users logged in:     0
  Memory usage: 5%                IP address for eth0: <ipaddr>
  Swap usage:   0%
Last login: Thu Dec  3 21:35:43 2020 from <local-ipaddr>
cumulus@ubuntu:~$