I did not check, if the crontab is written correctly, but if you want to periodically turn WiFi on and of, you need to place the command you have written in the /etc/crontabs/root file with the following command:
echo "25 6 * * 0-6 /sbin/wifi up" >> /etc/crontabs/root
To check if the command is in the crontabs list, execute the following:
crontab -l