Multiple concurrent WAN inbound

[freakalad]

I have a similar setup & problem to this fella.

I have my old existing ADSL connection - with a number of services & DNS's associated with that static-IP - and got a me a shiny new VDSL line (upgrading to fibre later), and a USB mobile PPP connection as a VPN "private backdoor" in the event any of the others go offline.

I've followed the docco guide dealing with fail-over & such, & set my own setup accordingly & seems to work OK, but this setup seems to only accepts & NAT's through traffic from the interface who's associated gateway is the present default one.
I've confirmed this via nmap, telnet, curl, etc via another mobile connection & toggling between default gateways.

In other words, effectively only a single interface is able to pipe in traffic, but not both simultaneously.

What I need to do is keep getting traffic in from my slower old connection piped through to my current servers, while slowly switching services over to my faster new connection.

What am I missing?