My network looks like this: http://imgur.com/yr3VUvl. I have tried the normal way of configuring nat, that is to set the ip nat outside on the serial towards ISP and ip nat inside on the serial towards R2. I have also permitted both the serial network and LAN network connected to R2. I have also added a default route towards the ISP and a static route from the ISP towards the HQ. But I am still unable to ping the ISP from the host.
I have this problem too.