Drupal 8: "Page cache maximum age" is not working for anonymous users
There was discussion in the chat, but there are still some things that can be answered.

The maximum age setting actually has not much to do with the built-in caching mechanisms of Drupal, as confusing as that is. It only controls the Cache-Control max-age header that is sent if the page is considered cacheable.

If you don't see a X-Drupal-Cache: HIT/MISS header in the response then that means your page is not cacheable. There are a number of things that can cause that, check if you have a session cookie (if yes, you need to figure out what's in there and who sets it), check that no module that you are using disables the page cache. For example captcha.module does that if you display a captcha as that can't be cached (not something like the built-in image captcha at least).

If you see that header then it is working. The Page cache modules actually don't care about that setting, they cache the page forever. However, any relevant change on the page automatically invalidates that page and fresh content is served. See https://www.drupal.org/docs/8/api/cache-api/cache-tags for more information on how that works.