Simple WAN watchdog for OpenWrt with LTE modem. Add to crontab.

reset_modem_hard.sh

#!/bin/sh
USB_GPIO=21
DELAY=1

gpio_value="/sys/class/gpio/gpio${USB_GPIO}/value"
echo 0 > ${gpio_value}
sleep ${DELAY}
echo 1 > ${gpio_value}

wan_watchdog.sh

#!/bin/sh

HOSTS="8.8.8.8 8.8.4.4 1.1.1.1 1.0.0.1"
PING_WAIT=2
SLEEP_TIME=5
NETWORK="wan"

tries=0
while [[ $tries -lt 10 ]]
do
        for host in ${HOSTS}
        do
                if /bin/ping -c 1 -W ${PING_WAIT} ${host} >/dev/null
                then
                        exit 0
                fi
        done
        tries=$((tries+1))
done

logger Ping watchdog triggered, restarting modem
ifdown ${NETWORK}
sleep ${SLEEP_TIME}
./reset_modem_hard.sh
sleep ${SLEEP_TIME}
ifup ${NETWORK}