Hello,
Even if Profibus and Modbus RTU use same bus technology (RS485), OLMs can not be used for Modbus RTU and mixing Modbus RTU and Profibus together will not work. One of the reasons (among many other) is a difference in timing.

Think about the following:

CPU315 PN ==> X104-2 ==> FO ==> X104-2 ==> ET200M for PN
                                         ==> FO ==>  X104-2 ==> ET200M for PM + CP341

The CPU act as PNIO controller.
It has two devices ET200M using an IM for Profinet IO.

The X104-2 is a switch with two FO ports.

The CP341 (with Modbus dongle) acts as Modbus Master and you start here a local RS485 Modbus Bus.

As alternative to the ET200M + CP341, you could think about a S7-1200 as i-device of the CP315PN.
Where the S7-1200 + CM act as local Modbus RTU master. (You need TIA)

Hello,

The X104 is a profinet switch (using Ethernet as standard).
The OLM is for Profibus.
Just connecting them with FO will not work
To express it simply, they use different langauges, and will not understand each other. .

I do not see why you could not use an ET200M directly connected on Profinet.
you just have to take another IM as head of that ET200M.
All other modules in the ET200M remain the same.

Hello,
I don't see any difference in this second attachment compared to the first post.

I already told you that this cannot work.
See my firts answer.