1) It might be that the PLC does not have a default gateway set, the Schneider might be sending UPD broadcast packets, which only work in LAN, for this, you can try to install and set up the UDP broadcast relay package. There currently is no wiki tutorial for this feature.
2) The test button is there mainly to check whether the configuration is correct, it does not appear in the data to server sent data. There is no log or anything to check what data is being read.
3) MQTT gateway allows you to send Modbus master requests to the router using MQTT messages and receive a reply. Data to server sends out data that was read using either Modbus RTU or TCP master. Either one of the functions can work independently.
For data to server, please check that you have entered correct connection string and remote port for AWS