I'm having similar difficulties. I use a Netgear DG814 ADSL Router / Modem which was fine in my little network (192.168.0.x). I added the e-smith server, originally server only, 1 nic, still fine. Then I tried to set it up as the gateway, using 2 nics. That's when my troubles began ..
My ISP is Eclipse in the UK and my router has been allocated a public IP address. The router supports PPPoA & I changed my internal network to 10.0.0.x and the External SME server IP to 192.168.0.2, leaving the ADSL router as 192.168.0.1. (Due to unrelated problems in changing the IP address in the router).
No luck & I've tried all combination of config settings. Should the Gateway IP address be my external IP address or the 192.168.0.1. I've tried both, but any suggestions as to where I should be double checking would be appreciated.