ATT Modem with SOHO in IP Forwarding Mode vs Passthrough?

We have an ATT Cellular Modem/Router that does not have any bridge mode capability, it hands off an Public IP to the WAN1 on the Peplink and handles all Routing and NAT (this cannot be changed on the ATT device). I want to setup my Peplink SOHO behind it but avoid double NAT to provide for VLANs, WiFi as well as utilizing the backup USB Internet and inControl remote access. Is IP Forwarding or IP Passthrough the correct method to do this. The public address from the ATT device is not static.
thanks in advance