source address translation

I am trying to configure my DIGI TRANSPORT WR21 to do source address translation but I am having difficulties, I would be appreciated if you could help me out.

The description of the case is as follows:

The digi interface 0 ip (WAN link) is and the interface 1 ip is

I want to make ip source address translation so that all the packets coming from Interface 0 with source: any and destination:10.200.12.x/24 are translated into source: and destination:10.200.12.x/24

This will help me reach network without configuring gateway on its devices.

I have searched and found that this could be done on DIGI Router using its firewall, yet I am not familiar with its syntax