Data exchange between 2 Delta PLC's DVP-12SE

I'm struggling with the following situation.

I'm using two Delta PLC's DVP-12SE.
I want to use the data exchange functionality between them.
That works perfectly when I connect the two PLC's with each other with a network cable and set up the data exchange on
one PLC (the master).

We would like to let the two plc's communicate with each other through Longe Range radio modules.
And this doesn't work.
I can receive the wireless send modbus message on the slave side, but it is not possible for the master to receive the modbus response back.

I wonder if there is more information available about the modbus specifications the DVP-12SE uses?
I wonder if there are timing issues we need to take care of?

Maybe somebody have heard about this specific situation before and maybe there is a solution for the problem?

Any advice into the right direction is highly appreciated!
I'm asking again if somebody has the experience to let 2 plc's communicatie with each other over modbus TCP/IP via radio modules.
I'm using the cheap modules of E-Byte: the E90-DTU 433C30E.

I really hope anybody can give me a hint in to the right direction.