NETMAP e.g. RUT300

Hello, i want to make a NETMAP from WAN to LAN and back.

The Config is the following:
Fritzbox 7590 (LAN-IP
Office network (
automation network (
PC (

I want to get access from the office network via Remotedesktop RDP to the PC.
The RUT300 should change the address to

i wrote the following to network->firewall->custom roles as a firewall custom role:
iptables -t nat -I PREROUTING -d -j DNAT --to-destination
iptables -t nat -I POSTROUTING -s -j SNAT --to-source

for example i have made the configuration in a router siemens scalance s615
(here only with another WAN-IP-address)

thanks for helping