cf. Ubuntu 20.04 Broadcom BCM4352 Wireless Network Adapter driver

Go to software & Updates, enable"Proprietary drivers for devices"

Reload sources, and apt-get upgrade

It'll grab the latest broadcom drivers and intel microcode, you'll need to disable secure boot if its enabled, reboot, and it should work.

Did it on my 9343 to 16.04 with kernel 4.4.0-21-generic

disabling Secure Boot Mode required

this case i set to :UEIF+NoSecureBoot