You are testing this truly from the WAN side, right? That is, you are testing thru your EdgeRouter from OUTSIDE your EdgeRouter.
Can you ping ANY hosts on our LAN when connected to your VPN?
Yes, your NAT should just be MASQUERADE on the WAN.
Am I correct that this worked with another router? Is your Pi running some flavor of linux? If so, you have ip_forwarding set, right?