Help putting a Balance 20 into operation with other firewall

Compared to many questions on here, mine is a very basic setup question. I’ve already bought a Balance 20 thinking it would suit my needs. Currently, I have a setup like this:

Internet --> DSL Modem in Bridge mode --> Firewall Appliance --> LAN

I have another consumer-grade connection like this:
Internet --> Cable Modem in Bridge mode --> ... But it’s not in use (yet).

I would like to set up the new network like this:

Internet --> DSL Modem -->  Peplink 20 --> Firewall --> LAN
Internet --> Cable Modem ------^^^

… so that I can balance between the two connections.

My question is how to go about this, but more specifically, how to avoid any sort of double-NAT happening. I am stuck trying to figure out what sort of network would then exist between the Peplink and the Firewall. Currently the Firewall is handling the PPPoE auth for the DSL, and I see in the Peplink setup I can also configure it.
I think that I know enough to be able to set up the WAN side of the Peplink, but the LAN side and that last hop between the Peplink and Firewall has me stumped.

Any help would really be greatly appreciated!

The Balance 210 and up supports drop-in mode which is an easy way to avoid a double NAT on your existing firewall and the primary ISP.

I did see that other models support drop-in mode, but I had already purchased the Balance 20.

Just disable NAT on your firewall appliance so there is only NAT on the Peplink and use IP forwarding instead. Add a static route on the Balance telling it that the firewall is the next hop for the LAN subnet behind it and you’re done.

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