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 172.20.30.42/19 and the interface 1 ip is 10.200.12.252/24.

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: 10.200.12.252 and destination:10.200.12.x/24

This will help me reach 10.200.12.0 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