Using a MultiPlus-II for automated grid simulation

We’re trying to simulate different grid situations with our MultiPlus-II. Therefore we want to change the AC-Out voltage and frequency. It has to happen via Node Red for it to be automated.
Currently I’m stuck at the point that I have the Modbus Nodes and the correct address for the frequency, but it won’t allow it to be changed.
I’m thankful for every tip or idea…

Are you grid connected?
If so, output is synced to the input.
What nodes are you using?
Changes would only affect the system when inverting.
You will need to add some more detail to your query, thanks.

Currently it’s grid connected with a Pylontech battery hooked to the DC-Battery connectors. We could change for a constant DC power source and disconnect the grid if necessary.
I’m using the node-red-contrib-modbus nodes and the adress from the VE Excel sheet.

Update.
I’ve changed the Inverter setting to Inverter only and deactivated the grid. I tried sending a value to address 4519, which should be AC-Out Frequency according to the VE Excel. I used FC 6 in the Modbus-Write comand. I got no Errors back, but also no change in the frequency.