Sorry the restriction also won't work on OLS since OLS doesn't support .htaccess syntax aside from rewrites. Again this will work in future as we will apply the restriction in the web server config rather than relying on .htaccess.
In the meantime you could do this with a .htaccess file inside the wp-admin directory using mod_rewrite syntax, something like:
RewriteEngine On
RewriteCond %{REMOTE_ADDR} !^1\.1\.1\.1$
RewriteRule . - [F]
