The extension allows you to communicate with byte/bytes without protocol, except for the implementation of ReadLine WriteLine with 7bit ASCII characters.
The serial servers that I have used are programmed to send/receive data bytes without protocol over the network, addressed to an ip/port and then convert it to a serial port.
The Modbus interface I mentioned is a separate extensions and implements Modbus TCP and RTU over Serial, BT and WIFI: se https://community.appinventor.mit.edu/t/modbusrtu-extension/
What do you need?