It is my understanding that you are trying to write an attenuation value by setting the 5-bits of the attenuator simultaneously. I also think that the timing of the control waveforms, i.e. the RF attenuation bits, may be critical as far as varying the RF power attenuation in realtime is concerned. However, there are no such methods/APIs as of R2019b under the MATLAB® Support Package for Arduino® hardware, which might provide a functionality to write the Arduino board's port pins simulatenously, i.e port write.
But I think you may consider a quick workaround using shiftregisters to write approproiate data to the RF attenuator, if it is not too much of a modificaiton to your hardware setup. Please refer to the documentation page on working with the shift register using MATLAB®: