I have a Fronius Symo connected to my MPii GX. I haven’t assigned a static IP addresses but doing so should be fine. My GX device could always see my Fronius Symo but it was never displayed on the VRM portal. This was fixed once I added the ESS assistant.
Your Modbus settings are correct. I did upgrade the firmware of my Fronius to 1.34. Also upgrade your Cerbo if it hasn’t been done recently. There was no setting required in the GX device. Hope you get this working as I know how frustrating it can be. Good luck.