Hi friend.
You could run the ont modify command to modify ONU sn.
This command is used to modify the ONT configuration data. When the ONT cannot be online in the normal state because the ONT configuration data such as authentication mode, SN, and password is different from the actual ONT configurations, you can run this command to modify such ONT configuration data to ensure that they are consistent with the actual ONT configurations.
For more details, please click: ont modify(distributing-mode)
Thanks.