You can find a similar example here: https://docs.telerik.com/devtools/aspnet-ajax/controls/grid/client-side-programming/events/onbatcheditopened.

There are two important bits to note:

• the combo box is generally not supported by the batch editing and if you encounter issues with its functionality, you can try to replace it with a RadDropDownList, or employ the following approach for extracting/populating data in a template: https://docs.telerik.com/devtools/aspnet-ajax/controls/grid/data-editing/edit-mode/batch-editing/working-with-templates.
• you must not make postbacks while using batch editing as this will destroy the current data (see here for more details: https://docs.telerik.com/devtools/aspnet-ajax/controls/grid/data-editing/edit-mode/batch-editing/server-side-api), only callbacks may be used for fetching data.