You doing routing on that firewall? Are there at least two inbound and two outbound interfaces on the firewall appliance?
If it is a multi in/ multi out firewall, you have a chance. You basically want the firewall to act as a bridge between your modems and the peplink. I don’t think it possible to combine separate WAN traffic on a single interface. I am sure someone will correct me if I am wrong. I don’t have a ton of experience with firewall appliances, but I know they come in all shapes and sizes - some with many logical components.
You may also be able to use the WAN VLAN tagging option and then set stuff up inside the firewall based on those VLan tags. Send VLan 400 to modem 1, send VLan 500 to modem 2, etc. that will put your firewall at the very edge and allow Peplink to still balance your outbound traffic.
Good luck, curious to see what you come up with!