You can use the Modbus RTU over TCP driver but it’s really a worst case scenario. You’ll have to make sure absolutely nothing else but Ignition is connected to these serial-to-ethernet converters. RTU over TCP is not a reliable protocol because there’s no framing or sequencing of requests. It’s possible for things to get out of sync and start seeing mismatched requests/responses.
Ideally you would use one of the Moxa MGate devices that actually convert Modbus RTU to Modbus TCP.