How to change apache error text if mod_proxy is used?

Question

linux

Good day everyone,
There is a proxy mod, and behind it a bunch of machines with sites, you need to change the text on one site that is issued to 403 error
on the proxy itself added to the host file

<Directory /var/www/html>
                Options Indexes FollowSymLinks MultiViews
                AllowOverride All
                Order allow,deny
                allow from all
</Directory>

added the .htaccess file to /var/www/html
and wrote the following there

Options + Includes

AddType text/html .shtml

AddHandler server-parsed .shtml

order allow,deny

deny from all

ErrorDocument 403 /var/www/html/403.html

I tried to register it on the host itself, but the result is 0,
I would be grateful to everyone for any help!