''I need a USB-RS232 converter to physically connect the dongle''
You mean it's a USB dongle? In that case, you would need to provide a USB Host for it. As Clive1 says, you would be far better off using a Zigbee module with a UART, SPI, or similar interface...
A complex system that works is invariably found to have evolved from a simple system that worked.A complex system designed from scratch never works and cannot be patched up to make it work.