Drop-in mode and WAN bonding

I have a simple question regarding drop-in mode, I have a topology the same as the below:

Now when the internal local users initiate a traffic through the firewall, the source IP of the local user will be Natted to
How the peplink will load balance the internet traffic to other ISP, like ISP2 or ISP3 although all the traffics coming from the firewall will have the source IP from ISP1 (

The Peplink will NAT the traffic coming from the firewall with is IP on WAN 2/3 before sending it to those WAN ISP

Thanks Rudy