Hello Kazimierz Szadkowski,
The web.config setting applies only to RadScriptManager. However you can enable CDN for Telerik controls when using ScriptManager only manually.
First you will need to get the CDN URLs of the required resources. To find them easily you can first run a page with the target control with a RadScriptManager with enabled CDN on the page and inspect the requested resource with FireBug or Fiddler. Then copy the needed URLs (you can see which scripts each control uses here
) and reroute them to their CDN counterparts as shown at the bottom of this help topic
This should do the work. I hope it helps.
All the best,
the Telerik team
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