Hi,
ok I have a 2830Vn serving my LAN, the current internet connection is Plusnet FTTC 80/20 connected to WAN2 via a BT VDSL modem, the service has 2 external IP's,
I also have a AAISP 3G service with a static IP for secondary MX with a USB modem connected to USB as WAN3 which is configured as fail over to WAN2
I have my exchange server at 10.*.*.11 and have configured open ports to route internet traffic from and to the server over the 2nd external IP which is also my MX.

this works fine and the 1st external IP is available to NAT on the LAN (the 2nd ext IP has been made un-available to NAT in WAN IP Alias section)
if I loose WAN2 the 3G service connects and all the LAN devices can communicate over 3G via the USB modem but the exchange server can not communicate using the 3G modem and does not see the internet, no matter what I try in open ports or Load-Balance/Route Policy sections I can not get the exchange server to see the internet via the 3G modem when in fail over?

Any help VERY much appreciated as its driving me nuts!!