Free UARTs TX and RX pins on CR-10S motherboard
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 (Raspberry Pi 3 Model B and B+) have onboard WiFi, or alternatively an older RPi (Raspberry Pi 2) could be used in conjunction with a USB Wifi dongle. On the RPi you could install a print server that talks directly with your printer over USB. One such an application that is frequently used is OctoPprint (the complete installation/image is referred to as OctoPi). 

You can then interface with the print server application using a web browser on any mobile device in your network, and if configured as such you could do that potentially from over everywhere over the world. The major advantage is that you then are not communicating G-code over WiFi as you would if you used RX/TX pins. Knowing that WiFi connection adds issues with latency and the possibility of a disconnection during the print, your fail rate would increase over a single board computer approach.

With an RPi the print will continue regardless of the Wifi connection.