PAT entry from Inside to outside

Hi Guys:

We have a WR21 in port isolate mode. Eth1 is the Internal network and Eth0 is the Internet via an ISP fiber or ADSL router. We are using GPRS as backup.
Eth0 is in DHCP mode so it picks up an ip address from the the ISP supplied router.
What I would like to do is this:
GRE tunnel end on the Digi is and I would like to translate -> which is the ISP router configuration page.(of
I tried used the static NAT setup but that appears to only work from the outside in whereas I’m going from the inside out.

Does anyone know how to do this?