Slicers don't talk to the printer. Slicers analyze an STL file and generate a GCODE file, based on your parameters. A print manager sends the commands from the GCODE file to the printer board, which executes them sequentially.
They are not. Commands are sent to the printer from a print manager of some sort. This varies among printers; some printers can be ...
Assuming you use Ultimaker Cura to slice, there is a MKS plugin that allows connecting to the MKS WiFi module that comes with the QQ. Just follow these steps:
Install the MKS WiFi Plugin
Click "Marketplace" in top right
Scroll down to find the "MKS WiFi Plugin"
Click on the plugin.
Restart Cura (quit and reopen)
Not answering the question directly which pins you can use I would like to propose an alternative solution for your problem, to explain why you should not use RX/TX pins.
An alternative solution includes the use of a small single board computer like the Raspberry Pi (RPi) which is connected over USB with the printer board. E.g. the latest range of RPi (...