Since you are using non-standard port names and the Modbus Binding does not use the openHAB serial transport (openhab-addons#7573) you need to set this non-standard port using -Dgnu.io.rxtx.SerialPorts=/dev/ttyUSB_DEVICE2
as documented in the Serial Port Configuration docs.
The Zigbee Binding does use the openHAB serial transport, so the openHAB serial transport will automatically add this configuration so it is possible to use this non-standard port name with the serial library.