Custom pages and templates not loading

I try to create a custom page for this page. But it will not load any static html. I have created a file called

page-pertest.php

but it seems that the index file overrules whatever I try to add.

As an alternative I tried to create a page template, but it would not show up in the Dashboard / Gutenberg.

The page I need help with: [log in to see the link]