Hi,
We run a very similar configuration, behind two routers and two firewalls, but do you need to use the WAN interface? We just run the UC5x0 without using the WAN port as anything that needs to communicate with the network is terminated on the firewalls.
Just use the internal LAN segment of the UC and skip the WAN interface.
We have several remote VPN sites and teleworkers and this configuration works for all of them,
Good luck
Bob