My Ubuntu 20.04.1 LTS wifi (no adapter found) on my Mac Mini (Intel i5-3210) was broken and I tried many things to fix it without luck.
This was the fix. It might work for you if you are having the issue. First go into settings, about, software updates and turn off the broadcom driver under ‘additional drivers’. Then open a terminal (Ctrl-T) and type :
sudo apt purge bcmwl-kernel-source wget http://mirrors.edge.kernel.org/ubuntu/pool/restricted/b/bcmwl/bcmwl-kernel-source_188.8.131.521%2Bbdcom-0ubuntu7_amd64.deb sudo dpkg -i bcmwl-kernel-source_184.108.40.2061+bdcom-0ubuntu7_amd64.deb