Simple bash script that allow to switch between wifi networks if internet connection down in your Raspberry Pi Zero.
Save the file and name it wifi-switch.sh for example. Make it executable by type in cmdline :
chmod +x wifi-switch.sh
Open crontab editor
sudo crontab -e
Paste the following line :
*/5 * * * * root /home/pi/wifi-switch.sh
Save the change by typing CTR+X
Test it and enjoy :)