Peplink 380 with MLPPP router plus cable modem

Hi folks, I have just ordered Peplink 380 and need to figure out how it is going to work with: an ISP-supported MLPPP connection (two DSL lines) plus a cable internet line. Here is the list of hardware in addition to the new Peplink 380:

  1. Microtik 750 router in MLPPP mode (this router is flushed with OpenWRT firmware and combines two DSL lines coming from two TP-link 8816 modems).

  2. a motorolla cable modem for a cable internet connection

  3. a Netgear srxn3205 wireless firewall/router (I no longer plan using Netgear device as the firewall and would rather use 380 as the firewall )

I know, most people would have figured this out first, and then ordered… but I hope that the entire thing is doable and then you can help me out with your suggestions.

The challenge is that Microtik router is very poorly suited for firewalling and I would rather use Peplink 380 or Netgear srxn3205 for this purpose. The big question is how to avoid double NATting (Microtik/OpenWRT router seems to require NAT and wont’s support MLPPP if put into a bridge mode. Has anyone uses MLPPP routers with peplink and if so how do you address the double-natting issue?

I drew a diagram showing the potential setup but I couldn’t find a way to attach it to this post.

So I have to describe this with words regardless of the fact that the picture worth a thousand of words.

DSL line1 -> DSL modem1 ->(WAN port )Microtik/OpenWRT(LAN port2)->(WAN1)Peplink380

DSL line2 -> DSL modem2 ->(LAN port1)Microtik/OpenWRT(LAN port2)->(WAN1)Peplink380

Cable line ->Cable modem -> (WAN2)Peplink380

Should Netgear Srxn3205 be connected on its WAN port to Peplink’s LAN port in order to act as wireless access point only (assuming I don’t want it to act as the firewall or do any NATting?)

How NAT should be configured on Peplink to avoid conflicting with Microtic? Is subnetting an option?

Please share your thoughts.