Allow reverse proxy access only from the host server

So the backend server is the target of the proxy, so the Proxy directive is not relevant there.

To restrict access to the backend, something like this in the virtualhost of your backend conf file would limit access to only the front-end server making proxied requests...

<VirtualHost *:80>
    DocumentRoot "/var/www/example.com"
    ServerName example.com
...
    <Location "/">
       Require ip 11.22.33.44
    </Location>
...
</VirtualHost>

You can add more granular controls with other directives from the manual:
https://httpd.apache.org/docs/2.4/mod/mod_authz_core.html#require