I followed this tutorial: http://www.techmonkeybusiness.com/hand-installing-pyserial-into-python-installations.html

The tutorial says:

Download the pyserial source code. The file you're looking for ends with .tar.gz
Unzipp what you've downloaded
Copy the serial folder it is under PySerial\pyserial-3.2.1\serial (the version can change)
Paste the folder in your blender modules folder 2.83\scripts\modules
and now you should be able to use import serial