How to access to property inside template repeater

This is not the correct way to get the templates from a repeater. You need to use the loadedTemplates property:

repeater = event.source.parent.getComponent('Template Repeater')
for template in repeater.loadedTemplates:

If the property you are trying to set is a custom property, you will probably need to dig it out of the template's dynamicProps field.

Here is an example from another post:
Update row in template repeater using button (outside from template) click