0 4   *   *   *    /usr/sbin/reboot

Adjust interval as needed.

Or if you want something a bit faster:

#!/bin/sh

NAME="$0"

logger_cmd () {
  echo $@
  logger -p daemon.info -t "$NAME[$$]" $@
}

if ! which ncat 1>/dev/null
then
  logger_cmd "ncat not found, installing..."
  opkg update && opkg install ncat
fi

chk_conn () {
  echo "Checking connectivity to $@"
  if ncat --send-only --recv-only -w 334ms $@ 2>/dev/null; then
    return 0
  fi

  logger_cmd "Cannot reach $@"
  return 1
}

restart_network_iface() {
    # TODO: Don't restart every minute

    COOLDOWN_LOCK=/tmp/internet-connectivity-watchcat.tmp
    COOLDOWN_SECONDS=300

    cooldown_time_end=$(cat $COOLDOWN_LOCK || echo 0)

    time_now="$(cat /proc/uptime)"
    time_now="${time_now%%.*}"

    cooldown_time_left=$((cooldown_time_end - time_now))

    if [ "$cooldown_time_left" -lt "1" ]
    then
        logger_cmd "Restarting network interface: \"$1\"."
        ifdown "$1"
        ifup "$1"

        cooldown_time_end=$((time_now + COOLDOWN_SECONDS))
        echo $cooldown_time_end > $COOLDOWN_LOCK
    else
        logger_cmd "Skipping interface \"$1\" restart due to cooldown. Cooldown left: $cooldown_time_left seconds"
    fi
}


logger_cmd "Checking internet connectivity..."

if   chk_conn google.com 443 \
  || chk_conn amazon.com 443 \
  || chk_conn facebook.com 443 \
  || chk_conn cloudflare.com 443 \
  || chk_conn telekom.de 443
then
  logger_cmd "Connected to internet."
else
  logger_cmd  "Not connected to internet."
  restart_network_iface "$1"
fi

In restart_network_iface use /usr/sbin/reboot instead of interface up/down and run the script every few minutes via cron or systemd timer. This was written for OpenWrt so if you use that you can use it as-is. For other systems you’d also have to adjust the logger_cmd.

You can place that on another machine and send a signal to a smart plug instead if you’re worried of a locked up / frozen router. That said if your router freezes like that, you should probably change it and you should be able to run this script on it.