Apache behind a Reverse Proxy in DataPower

Question

I have been checking this previous question Apache Virtual Host SSL Behind Reverse Proxy that seems to be similar to what I am looking for.

The main differences are two:

1. I am not using SSL (for now)
2. DataPower is the reverse proxy before the Apache HTTP Server.

I have the following config on Apache:

<VirtualHost *:7777>
   ServerName testsite.example.com
   Port 80
   ServerAdmin you@your.address
   RewriteEngine On
   RewriteOptions inherit
</VirtualHost>

<VirtualHost *:7777>
   ServerName testserver-01.example.com
   Port 7777
   ServerAdmin you@your.address
   RewriteEngine On
   RewriteOptions inherit
</VirtualHost>

<VirtualHost *:7777>
   ServerName testsite.example.com
   Port 7777
   ServerAdmin you@your.address
   RewriteEngine On
   RewriteOptions inherit
</VirtualHost>

<VirtualHost *:7777>
   ServerName testserver-02.example.com
   Port 7777
   ServerAdmin you@your.address
   RewriteEngine On
   RewriteOptions inherit
</VirtualHost>

DataPower is load balancing testserver-01 and testserver-02 and also proxies them publishing the testsite.example.org hostname in port 80.

Since the web application uses HTML Login Forms, DataPower does not follow redirects, to avoid the request for /j_security_check getting rejected.

But, although the option "Rewrite URL" is set as On, every time we get into the web application we end up at the testserver-01 IP address, jumping DataPower in the request.

I need to get that fixed, so DataPower ALWAYS takes part in the request flow, through testsite.example.com

Is there maybe something wrong with the Apache httpd.conf?