You can configure one of the computers as a Key Management Service (KMS) host and activate the KMS host by phone. The other computers in the isolated network can then activate using the KMS host.
Installing a KMS host key on a computer running Windows 10 allows you to activate other computers running Windows 10 against this KMS host and earlier versions of the client operating system, such as Windows 8.1 or Windows 7. Clients locate the KMS server by using resource records in DNS, so some configuration of DNS may be required. This scenario can be beneficial if your organization uses volume activation for clients and MAK-based activation for a smaller number of servers. To enable KMS functionality, a KMS key is installed on a KMS host; then, the host is activated over the Internet or by phone using Microsoft's activation services.
Reference:
https://docs.microsoft.com/en-us/windows/deployment/volume-activation/activate-using-key-management-service-vamt