If you find that your Mac Mini's wireless card is not working correctly after installing Linux Mint (mine was not displaying available wireless networks) try these steps:
first make you you've got the right driver (my Mac Mini needs the Broadcom BCM4321 802.11a/b/g/n driver):
sudo apt update
sudo apt install firmware-b43-installer
sudo reboot
If that doesn't resolve the problem try switching drivers (in my case the wl driver seems to be the problem):
sudo modprobe -r wl
sudo modprobe b43
Check to see if wireless networks are displayed, if so then add the wl driver to the blacklist:
sudo touch /etc/modprobe.d/blacklist.conf
sudo echo "blacklist wl" >> /etc/modprobe.d/blacklist.conf
The b43 should then become the default driver.
You may need to manually edit the blacklist file with elevated privileges
eg:
# blacklist wl driver as it wont list available wireless networks
#https://www.linuxquestions.org/questions/linux-networking-3/ubuntu-16-04-mac-mini-2009-broadcom-driver-selected-and-wlan-enabled-but-home-network-does-not-show-4175600634/
blacklist wl
See original article
here.