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.
2901 (r2) needs a default route towards 2901 (HQ) and 2901 (HQ) needs a default route towards the ISP. Also 2901 (HQ) needs a static route for PC and server subnet pointing to 2901 (r2).
You should have something like that on your 2901 HQ router
ip address 10.0.0.3 255.255.255.0
ip nat outside
interface serial 0/1
ip address 192.168.1.1 255.255.255.0
ip nat inside
ip nat inside source list PRIVATE-NETS interface Serial 0/0 overload
ip route 0.0.0.0 0.0.0.0 10.0.0.1 name INTERNET
ip route <internal networks> <subnet mask> <IP of R2's next hop>
ip access-list standard PRIVATE-NETS
*To test you could create just a loopback on ISP router and nothing else, example:
interface lo 100
ip add 18.104.22.168 255.255.255.255
* Remember to use routing betwen HQ and R2 in order to advertise a default route and know how to reach the internal networks.
Hope it is useful