I have heard 3 possible ways to fix it. I have only tried the first.
1) Use OpenVPN instead. You only need one UDP or TCP port open in order to use this. Default is usually 1194 UDP. If you can use IPSec then surely 500 UDP must be available.
2) I believe you can configure IP Passthrough in the Centro Grande. Install a firewall on your VPN server (using iptables or ufw if you are running linux), open the relevant ports and I think this will resolve the problem. Note that for this to work you must use a different server as a router, and if you misconfigure your firewall you will be open to attack. It is not a simple option.
3) You could try a different router, such as the Fritzbox.