OnPrem Gateway New Data Source Error: Service Account failed to impersonate the user

Thanks Anonymous . I had found that post and see quite a variety of scenarios/solutions. I've reviewed again and since managed to do the following:

1. Create local AD domain account: DomainName\PowerBIAdmin
2. Added DomainName\PowerBIAdmin as Windows Administrator on the Gateway server, and configured the OnPrem Data Gateway service to run under this account
3. In Local Group Policy (gpsedit.msc) on the Gateway server, I've added the DomainName\PowerBIAdmin account to [Impersonate a client after authentication]. 
4. Create Azure AD account: PowerBIAdmin@DomainName, with same password as local AD domain account (I'm naievely working on the hope that having the same Name and Password across the local AD and Azure AD accounts will somehow help 😅)
5. Configured the Azure AD account as a server administrator of Azure Analysis Services
6. Restarted the On Prem Data Gateway service

When I now try to create the new Datasource in PowerBI Service,, I use the Azure AD account (PowerBIAdmin@DomainName) and now receive the following error:

GWY_Cellnet: An invalid connection string has at least one of the passed arguments which does not meet the parameter specification. Please check the data source connection string.
Underlying error message: Authentication failed: User ID and Password are required when user interface is not available.