Port forwarding RUT240

Hi, I like to go familiar with the port forwarding at RUT units!
So I tryed everything from the manual and wiki I could found but it’s doesn’t work!
My unit is a RUT240
Firmware 7.06.06 (the actual! )
Both devices (SPS and Modbus Master) are in different ip ranges!

In a first step I like to connect my modbus master at the wan interface to my SPS connected at the lan interface! Is there a howto were all steps are explained?
In a second step I like to to the same with mobile interface!
Please can you help me?