Can the following article help?
ASP.NET 4.5 ScriptManager Improvements in WebForms
https://devblogs.microsoft.com/dotnet/asp-net-4-5-scriptmanager-improvements-in-webforms/
In ASP.NET 4.5 or later version, for the server controls which use client script to work properly, the required scripts need to be registered to the ScriptManager and the pages should include the ScriptManger.
[Edit (added)]
See the following article if you use the Ajax Control Toolkit.
How to Use Bundling and CDN
https://github.com/DevExpress/AjaxControlToolkit/wiki/How-to-use-bundling-and-CDN