An internal subnet address as a gateway?

I recently managed to get my FBIII/700 and Watchguard v7.0.B1.366 up
and running on my 192.168.x.x LAN, but I still have a configuration
error/problem I think and no external access. My satellite ISP, at his
NOC, maps my three public (WAN) IP addresses to three 10.x.x.x
addresses which he provides at my router. Therefore, if I want to have
incoming mail, for example, routed to a mail server on my DMZ (Optional
interface) subnet, that mail server has to have a 10.x.x.x series
address. Also, my Satellite router too must have a 10.x.x.x address and
therefore the problem I am having is, I think, that the Watchguard
v.7.0.B1.366 software I have is not going to let me use an 'internal'
subnet address as a gateway, and so I cannot access the internet. Is
that right and is there a workaround?

Terence.