Hi Neri
The way this should work is that the client connects to the guest network and gets an IP address from DHCP. The DHCP configuration should include the default gateway and must include a DNS address.
When the client opens a web browser the browser tries to connect to the configured home page. This means that a DNS lookup is sent out and the controller intercepts it and forwards it on. Providing there is a response from the DNS server the controller will cause the client browser to re-direct to the web authentication login page.
It is therefore essential that the controller can see the DNS server. Forget the PING for now - DNS is a must. You can prove the rest of the system by ensuring the guest client has an IP address. Open the client browser and try and connect to http://1.1.1.1 (assuming your virtual interface on the controller is 1.1.1.1). If you get re-directed to the web authentication login page then the issue is a DNS issue.
Regards
Roger