Thanks for using Syncfusion products.
Query: "How can I filter custom columns? "
From your query, we understand that you need to filter the template column. Operations like filtering, sorting, grouping will be based on the field value of column property. From your code example we could find that you have not defined the field property to template column. So we suggest you to define the field value to column. Based on the field value column will filtered or sorted etc.
refer the below code example
<e-column field="vorname" headerText="Fullname">
<ng-template e-template let-data>
Refer our UG Documentation note section for your reference
Please get back to us if you have further queries.