From what I have read so far, it is not possible to use the dongle if one uses Modbus RTU. But maybe someone has figured out a way to do this.
I’m currently using the Dongle together with two LAN to Wifi bridges connecting to the two internal APs of my two inverters. This used to work perfectly until the end of last year. But now I get frequent Modbus connection errors with the integration. Very annoying. So I would also love to switch to Modbus RTU, but since I’m running a setup with cascaded inverters, I (unfortunately) have to use the dongle.