Reference:
Regions and Availability Domains Volumes are only accessible to instances in the same availability domain . You cannot move a volume between availability domains or regions.
FYI: https://docs.cloud.oracle.com/iaas/Content/Block/Concepts/overview.htm

To accept traffic from the internet, you create a public load balancer. The service assigns it a public IP address that serves as the entry point for incoming traffic. You can associate the public IP address with a friendly DNS name through any DNS vendor.
A public load balancer is regional in scope. If your region includes multiple availability domains, a public load balancer requires either a regional subnet (recommended) or two availability domain-specific (AD-specific) subnets, each in a separate availability domain. With a regional subnet, the Load Balancing service creates a primary load balancer and a standby load balancer, each in a different availability domain, to ensure accessibility even during an availability domain outage. If you create a load balancer in two AD-specific subnets, one subnet hosts the primary load balancer and the other hosts a standby load balancer. If the primary load balancer fails, the public IP address switches to the secondary load balancer. The service treats the two load balancers as equivalent and you cannot specify which one is "primary".
Whether you use regional or AD-specific subnets, each load balancer requires one private IP address from its host subnet. The Load Balancing service supplies a floating public IP address to the primary load balancer. The floating public IP address does not come from your backend subnets.
You cannot specify a private subnet for your public load balancer.
The backend servers (Compute instances) associated with a backend set can exist anywhere, as long as the associated network security groups (NSGs), security lists, and route tables allow the intended traffic flow.
Oracle recommends that you create your load balancer in a regional subnet.
Oracle recommends that you distribute your backend servers across all availability domains within the region.