Specify the view mode when the node is displayed on modal window (ajax)
The way you are doing it will always show the full view mode, as you are linking to the canonical entity page - which is the page to see the full view mode. If you want to show a different view mode, then I would suggest creating a custom route (or path if you're using D7), and in the callback, generate the output programmatically with the view mode you want. I'd probably use the following for the path:

node/[NID]/teaser


And in there I would output the teaser view mode.