DNN 8 and 9 are trying to get rid of the Telerik components, but not everything has an adequate replacement. So if you need to re-enable them, here's a short explanation.
Why Does DNN try to Remove Telerik?
Around 2009 DNN entered a strategic OEM partnership with Telerik. The idea was that DNN could provide better tools to developers and Telerik could reach more developers. DNN aggressively started using the components - resulting in many parts only working if Telerik parts were also in the installation. Later on it turned out to be a bad idea an DNN & Telerik ended the partnership (see also this blog post). But there was a problem: many DNN parts had to be re-developed from scratch - and nobody has time for refactoring.
So DNN is trying to do a phase-out. In DNN 8 and 9 some Telerik components are still installed, and others are included for manual activation if needed.
Rumors have it, that DNN 9.2 will completely free itself from Telerik. But I'll believe it when it's here, as this has been rumored before, and never worked out. A core component which still uses the Telerik libraries are the File-Picker UIs, and as of now (June 2017) there is still no replacement yet.
Why does 2sxc need Telerik?
2sxc sometimes provides a file-picker to let users choose a file. DNN has such a picker, but it's part of the WYSIWYG RAD-editor (so part of the Telerik distribution). So if you run 2sxc and try to browse for files, you might see this dialog:
The dialog message: can't connect to the dialog - you are probably running a new DNN (v.8+) and didn't activate the old Telerik components. Please install these in the host > extensions to get this to work
Two Telerik Components
What has been confusing is that some Telerik components are activated, leading people to believe that everything is already enabled. But the activated parts are only certain libraries, not the editor / file-picker.
Activating the Telerik RadEditor
If you go the host section and go to available extensions, you'll find a section called Providers. Open this, and you'll find a RadEditor Manager. This is what you must enable:
Detecting that it worked
Once this is enabled, the RadEditor could be used (but by default you'll continue to use the CKEditor in DNN, wheras 2sxc uses the TinyMCE by default anyhow). But now the file browser will work again, and will look like this:
Love from Switzerland,
iJungleboy