Site's 404 error message not accessed

I created a site specific 404.shtml page, however the 000webhost.com generic 404 is shown with 404 errors. This has no return to the previous page or home page links.

I tried to modify the .htaccess in the public folder without success.
To the existing text:

HTID:7360251: DO NOT REMOVE OR MODIFY THIS LINE AND THE LINES BELOW

php_value display_errors 1

DO NOT REMOVE OR MODIFY THIS LINE AND THE LINES ABOVE HTID:7360251:

I added:
RewriteEngine on
ErrorDocument 404 /404.shtml

Any ideas how to have the sites specific 404.shtml would be welcome.

Why .shtml? Change the extension to .html