With a drop-in mode deployment you have an advantage of keeping public IP addresses on your firewall. The firewall needs to do a NAT for that scenario and internal networks are not visible to the Peplink.
If the firewall has additional public IPs it may be possible for each internal network to have a unique public IP and the Balance could recognize the source.
Other options are to identify the destination or traffic type instead of the source. You could also deploy the Balance in NAT mode turning off NAT in your firewall. With that deployment LAN static routes get added to the Peplink and internal LAN networks/clients are visible for outbound policy rules to work when identifying the source.
The Balance also has a stateful firewall and it can do VLANs for internal networks so removing the existing firewall may be another option for you.