Reference:
Can I set up Data Guard across Availability Domains? Yes, you can set up Data Guard in the same or different Availability Domains in a region. However, Oracle recommends that you set up your Data Guard configuration across Availability Domains. Can I set up Data Guard across Oracle Cloud Infrastructure regions? Yes, you can set up Data Guard across regions,
"but the Database Cloud Service Data Guard feature currently does not support it. " You can manually set up Data Guard across regions by logging on to your host and using DGMGRL. You must enable an internet gateway on the primary and standby DB system VCN for Data Guard to transport logs across regions. Learn more about DGMGRL.
To configure a Data Guard system across regions or between on-premises and Oracle Cloud Infrastructure DB systems, you must access the database host directly and use the DGMGRL utility.
https://docs.cloud.oracle.com/iaas/Content/Database/Tasks/usingdataguard.htm