As stated in MAAS documentation, altering the preseed files is not recommended, because it might prevent MAAS from properly commissioning and installing nodes.
Are you able to connect to the node using SSH and user
ubuntu? If so, looking in
/var/log/syslog can help diagnosing startup and networking issues.
If you need to alter the preseeds, you can find the full documentation, describing all supported settings here. There is a section about changing the default root user account and password (see "Account setup"). If you change these though, MAAS might have trouble bringing the node up to Ready state.