MODBUS TCP is not supported. Digi does not have a Modbus TCP port for Netos. If it is using standard TCP sockets it should be fairly simple to port. You have to figure out what APIs your Modbus implementation requires and figure out which one of those are available in Netos.
For Embedded Linux you could use some of the existing projects out there (I am afraid that they are not completed or well maintained/supported), or buy some commercial library like this one: