How do I render an entity view mode like 'teaser' in a twig template?
The best practice would be to configure the reference field in UI as teaser and use it in the node template:

{{ content.field_entity_ref }}


If this is not an option for your case, you would probably need some code in preprocess.

But much easier is to use the module Twig Tweak, which can render nodes without preprocess:

{{ drupal_entity('node', node.field_entity_ref.0.target_id, 'teaser') }}