L2TP/IPSec VPN on Fortigate which is behind a Velocloud Edge Device for SD-WAN

Working with a FortiGate that previously had a L2TP/IPSec VPN for Dial-up/Remote users configured. The device now sits behind a Velocloud Edge SD-WAN device and the WAN connection is plugged into it with an uplink from the edge device into WAN1 port on the Fortigate configured with a static LAN IP. Now from the Velocloud, they have setup a 1:1 NAT for the public IP that was once configured on the Fortigate for this traffic {PUBLIC IP --> FORTIGATE through the Velocloud device}, VPN connection fails in Phase 2 based on what I see within the logs. My thoughts, is that I need to add a Secondary IP to the WAN1 configuration on the Fortigate of the public IP address and configure that as the local gateway within the IPSec Tunnel network configuration. Just want to get some thoughts from the community on this.