The Reading settings are not for summaries on your page, they're for summaries of your posts when read from an RSS reader.

There is a way to do this in the coding, but I don't recommend messing
about with coding if you're not familiar with it.

Here's an easy fix: In order to get your posts to show only summaries on the main page, you will need to add a summary in the "excerpt" section on each individual post. Directly below the body section, where you wrote or pasted the entire post, is an "excerpt" box. Place a summary of the post there.