If I understand what you're saying, you want to do this:
Code:
DSL modem <--> Grandstream (bridge mode) <--> Router (PPPoE & DHCP)
The Grandstream won't be able to access the Internet in that fashion because the PPPoE is happening on the router (behind it). Since you have to use PPPoE, I believe your only option for putting the Grandstream ahead of your router would be to configure it for PPPoE instead of your router and leave it it routed mode. Then your public DSL IP address would be on the Grandstream and you would use a private subnet (192.168.x.x) between the Grandstream and your router with a different private subnet behind the router.
Basically something like this:
Code:
DSL modem <-----------> Grandstream <-----------> Router
PPPoE No PPPoE
Public IP on WAN side Private IP from Grandstream LAN on WAN
Private IP on LAN side Separate private IP on LAN side
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