Edit the page. Then on the right side of the editor, below the “Publish” or “Update” button, there should be an option to change Template. Choose the 0 Sidebar option on the drop-down list then update the page.
Page Templates only work on Pages and not on Posts.
There’s not even a Default Sidebar on the list. That’s probably from the plugin you’re using, that’s not part of the theme. You’re probably looking in the wrong place. The area is titled Page Attributes. If that box is not available, click the “Screen Options” on the top right corner of the editor page.
The only options available on Frontier Page Templates are: Default Template 0 Sidebar, Full 1 Sidebar, Left 1 Sidebar, Right 2 Sidebars Blank Sitemap
You could also change page templates using the Quick Edit option on “All Pages” section of the admin area.