IpSec between HQ Load Ba-lancer device towards Fortinet

[Closer01]

Hi Guyz,

Need your opinion here, Im stuck on how to implement the Ipsec Site-tosite VPN between HQ and Branch office as per diagram image attached. The issue that I'm facing is from HQ site, in front of the firewall, the is an Accelink load balancer which carry 3 ISP gateway to internet, and the connection to fortiner (HQ) firewall, is configure as a private IP. Ive tried to establish a IPsec between branch and HQ , but from branch FW it can only see up to firewall HQ and not the LAN segment behind the HQ firewall. So my question is.

1. How should i establish a IPsec VPN configuration if infront of the HQ firewall is a Load Balncer device?

2. There is an option of NAT-T on the HQ FW, but the option didnt have in HQ Load Balancer.