Call SharePoint Server Subscription Edition REST API via App Only Authentication

Need to call SharePoint Server Subscription Edition REST API via App Only Authentication and found below article.

https://learn.microsoft.com/en-us/sharepoint/dev/solution-guidance/security-apponly-azureacs

can i do this using TokenHelper.cs as suggested in the article? 

Seems request tires to access ACS because of below settings in TokenHelper.cs and requests timeout since the code is executed from a Server where no access to Azure ACS.
private static string GlobalEndPointPrefix = "accounts";
private static string AcsHostUrl = "accesscontrol.windows.net";

Any support on how to do this for On-Prem SharePoint is really appreciated.