One of my TV Boxes has broken WiFi and I have installed a USB 2.4/5 GHz WiFi adapter without major problems. Everything works better than before. The only care to keep in mind is that the USB WiFi adapter is supported by CoreELEC and disconnect the inbuild WiFi driver that does not work in /storage/.config/modprobe.d/blacklist.conf.
In my case the driver for the USB WiFi adapter is 88x2bu, and my inbuild driver to disconnect is wlan (the content of my blacklist.conf is blacklist wlan). You will see your drivers with the lsmod command in SSH.