Try whitelist VPN-assigned internal IP addresses in your .htaccess
SetEnvIf X-Forwarded-For "^10\.8\." VPN
SetEnvIf X-Forwarded-For "^192\.168\." VPN
<RequireAll>
Require all granted
Require env VPN
</RequireAll>
Note: If you're not using a reverse proxy or the X-Forwarded-For header is not being set correctly, you may need to adjust the code accordingly or investigate other options for passing the client's internal IP address to Apache.