You should normally be able to access group fields within a repeater from the Dynamic Selector.
It isn’t currently possible to refer to a repeater group field through the dynamic inserter UI, but you should be able to retrieve the field value by using the following: {acfrepeater=your_group_name=false=group_field_name}
@Louis, just a quick follow up, without frontend rendering on, this works flawlessly. With frontend rendering on, it is coming back as [object Object].
Without frontend rendering:
With frontend rendering:
Show in REST API is enabled and other fields work correctly, so it is specific to these Repeater + Group fields.