unable to establish Wi-Fi connection on some wireless networks
I believe the Broadcom STA driver is incorrect for your device 14e4:4727. Please get a temporary wired ethernet connection, open a terminal and do:
sudo -i
apt-get remove --purge bcmwl-kernel-source
apt-get install linux-firmware-nonfree
echo brcmsmac >> /etc/modules
exit
Reboot and your wireless should be operating correctly.