Hi @DrCheese 
Thanks for posting in our business forum.
If so, you cannot touch the settings on B and C, there isn't too much we can do about it.

Based on the diagram, B and C should be located in another LAN which means there is another router.

Unless you can make sure A and D are under the same subnet and no ACL on B and C and this "another router", you should get them working without configuring anything at all.

Make sure you have port forward on A and D, or open up the router page for them. They should work without any routing.

  @DrCheese 

I think you are running into the problem of the TPlink routers being gateways, not routers.  Currently you cannot turn off NAT on these devices.

A simple test to prove this out is to set PC1's IP as the DMZ IP of Router A and set PC2's IP as the DMZ of Router D.  PC1 should now be able to see PC2 as the WAN IP of Router D,  and vice versa for say file sharing or http access.

One way to fix this problem is to use VPN connections between Routers A and D.  This will limit the bandwidth at some point, but you get routing in return. You have better options on the ER605v2 than i have on the original ER605, but this is just a test, you can optimize with other VPN tech later.  Since you have public static IPs on both WAN interfaces of the ER605, you can use the built-in IPsec "branch to branch" option, so configure that on Routers A and D.  After a reboot, you should have full routing between the LAN subnets of Router A and Router D.

A different TP-link router will not help, they all have the same basic software architecture, the only difference is increases in CPU/memory and ports.

You can only have 1 DMZ IP per WAN port.  IN layman's terms, it basically forwards any port not already explicitly forwarded via a rule to that IP as default.  If you have access to addtional IP's on the WAN side, you can configure multiple WAN ports on the TPlink ER605 (or any gateway), each with it's own unique WAN IP and DMZ IP.  You will likely require a simple ethernet switch to provide enough ports to physically cable up 1-4 WAN interfaces.

In my experience, pt-pt links such as I suspect you have, have management IP's, but can also be L2 transparent, meaning you can run the same subnet on either side of the link and get away from all of the NAT/DMZ stuff.   Are any of the devices connected to the internet, and if so, how?