Configure Multiple WAN through one interface

Hi!
I'm trying to configure a multiwan environment through one external interface.

Attached is a diagram of what I'm trying to achieve.

On the external interface I'm able to configure one of the routers WAN address and connecto through it, but I would need to be able to configure the other WANs.
It is possible to define additional addresses for the external interface but those additional addresses do not have the possibility of configuring a gateway.

Any help will be appreciated.

Regards 
Sergio G

This thread was automatically locked due to age.