The .NET DataStore does not limit you only to use REST... you could use SOAP, GraphQL, etc.  .NET DataStore is just a data object, and what you do with the data depends on the C# code you write to handle.

First, before you embark on this conversion project you need to have strong C# programming skills.  If you don't have such skills, then I recommend you engage one of our consulting partners.

At a high level, what you need to do is the following (with PowerBuilder/SnapDevelop 2019 R3 or newer):

1. Use the PowerScript Migrator to convert the PowerScript NVOs to C# classes. 

2. Create SOAP/XML interface for the C# classes. There are number of ways to do this for .NET Core 3.x, and here is just one example.

3. In the PowerBuilder client app, use the HTTPClient object to call the SOAP Web service.

I'm sorry, but Appeon doesn't have SOAP examples to provide you.  SOAP is not very popular these days.