I got it but what will happen to my server that are published on various internet connection since the connection between the firewall and the peplink is only one WAN connection and the rest are connected in Peplink Balance. Please check the link below for your reference network topokogy.
The firewall uses a single outside interface with a drop-in mode deployment. The other WAN connections will get a NAT from the Peplink to the outside IP address(es) of your firewall. The firewall uses the same default gateway of the primary ISP router.
Two steps are needed for this:
a) LAN static routes are configured in the Balance for the HO internal networks, pointing to the outside interface of the firewall.
b) The Firewall has NAT exemption policies so there is no NAT when source = internal networks and destination = branch office LAN. The NAT exemption policies are also configured for the reverse direction.