An example failover script for dual WAN, using a ping healthcheck and managing default routes appropriately

failover.sh

#!/bin/bash

# Set defaults if not provided by environment
CHECK_DELAY=${CHECK_DELAY:-5}
CHECK_IP=${CHECK_IP:-8.8.8.8}
PRIMARY_IF=${PRIMARY_IF:-eth0}
PRIMARY_GW=${PRIMARY_GW:-1.2.3.4}
BACKUP_IF=${BACKUP_IF:-eth1}
BACKUP_GW=${BACKUP_GW:-2.3.4.5}

# Compare arg with current default gateway interface for route to healthcheck IP
gateway_if() {
  [[ "$1" = "$(ip r g "$CHECK_IP" | sed -rn 's/^.*dev ([^ ]*).*$/\1/p')" ]]
}

# Cycle healthcheck continuously with specified delay
while sleep "$CHECK_DELAY"
do
  # If healthcheck succeeds from primary interface
  if ping -I "$PRIMARY_IF" -c1 "$CHECK_IP" &>/dev/null
  then
    # Are we using the backup?
    if gateway_if "$BACKUP_IF"
    then # Switch to primary
      ip r d default via "$BACKUP_GW" dev "$BACKUP_IF"
      ip r a default via "$PRIMARY_GW" dev "$PRIMARY_IF"
    fi
  else
    # Are we using the primary?
    if gateway_if "$PRIMARY_IF"
    then # Switch to backup
      ip r d default via "$PRIMARY_GW" dev "$PRIMARY_IF"
      ip r a default via "$BACKUP_GW" dev "$BACKUP_IF"
    fi
  fi
done

Instead of deletion and addition of routes, we can also change interface Metric, which is good.
I write a script for openwrt below as mwan3 not working properly with my VPN, so need to create my own alternative

set -e
CHECK_IP=${CHECK_IP:-8.8.8.8}
PRIMARY_IF=${PRIMARY_IF:-prmwan}
BACKUP_IF=${BACKUP_IF:-bckwan}
ACTIVE_METRIC=5
FAILOVER_METRIC=10

HEALTH_CHECK_FAIL=2

# Check if number of ping failed then retun false with interface
is_up() {
  network_name=$1
  interface=$(get_interface $network_name)
  if ping -I "$interface" -c$HEALTH_CHECK_FAIL "$CHECK_IP" &>/dev/null
  then
    echo true
  else
    echo false
  fi
}

is_primary_up() {
  is_up $PRIMARY_IF
}

is_backup_up() {
  is_ip $BACKUP_IF
}

# Get interface from network name
get_interface() {
  network_name=$1
  interface=""

  if [ $(uci get network.$network_name.proto) == "pppoe" ]; then
    interface=pppoe-"$network_name"
  else
    interface=$(uci get network.$network_name.device)
  fi
  echo $interface
}

activate_primary() {
  primary_metric=$(uci get network.$PRIMARY_IF.metric)
  backup_metric=$(uci get network.$BACKUP_IF.metric)
  if [ "$primary_metric" != "$ACTIVE_METRIC" ] || [ "$backup_metric" != "$FAILOVER_METRIC" ]; then
    uci set network.$PRIMARY_IF.metric=$ACTIVE_METRIC
    uci set network.$BACKUP_IF.metric=$FAILOVER_METRIC
    uci commit network
    /etc/init.d/network reload
  fi
}

activate_backup() {
  primary_metric=$(uci get network.$PRIMARY_IF.metric)
  backup_metric=$(uci get network.$BACKUP_IF.metric)
  if [ "$backup_metric" != "$ACTIVE_METRIC" ] || [ "$primary_metric" != "$FAILOVER_METRIC" ] ; then
    uci set network.$BACKUP_IF.metric=$ACTIVE_METRIC
    uci set network.$PRIMARY_IF.metric=$FAILOVER_METRIC
    uci commit network
    /etc/init.d/network reload
  fi
}



if [ "$(is_primary_up)" = "true" ]; then
  activate_primary
else
  if [ "$(is_backup_up)" = "true" ]; then
    activate_backup
  fi
fi