Minimum firmware: 18.4.54

Create a new custom script under System -> Scheduled tasks -> custom scripts, and enter in the following.   Adjust the Interval to the desired interval you would like this script to run.

# Test the WAN connection with a ping test.  Keep a count of failed tests.
# If this test fails for three times in a row, then bring down the WAN link.

# Adjustable settings
test_server='128.136.167.120'  # where we perform the ping tests to
fail_count_file='/tmp/custom_wan_test_fail_count.txt'
fail_count_limit='3'  # number of concurrent failures that can occur

test_failed() {
  try=$((try+1))
  echo "$try" > "$fail_count_file"
  accns_log w config "custom: wan test failed ($1 - try $try)"
}

test_passed() {
  rm -f "$fail_count_file"
  config set network.interface.wan.ipv4.metric 1 # raise WAN priority
  try=0
  # Note: uncomment the following line if you want to log successful tests
  accns_log w config "custom: wan test passed"
}

ping_test() {
  # 3-attempts, 5-second timeout, 1-byte packet size
  /etc/netmon/netmon_test.sh wan "$(runt get network.mark.ipv4.interface_wan)" 3 5 ipv4 ping "$test_server" 1
}

### MAIN ###
try=$(cat "$fail_count_file" 2> /dev/null)
try=${try:-0}

# make sure $try is an integer. set to zero if not
case $try in
  ''|*[!0-9]*)
    try=0
    ;;
esac

# do the connectivity test
if ! ip link show dev wan | grep -q 'state UP'; then
  ip link set dev wan up
  sleep 5
fi

if ! ip route | grep -q "dev wan"; then
  test_failed "No WAN connection"
elif ping_test; then
  test_passed
else
  test_failed "ping failure to IP $test_server"
fi

# lower WAN priority if failed test count is greater than specified limit
if [ "$try" -ge "$fail_count_limit" ]; then
  # note, that we don't reset the fail count. If we fail next attempt, lower the WAN metric again.
  config set network.interface.wan.ipv4.metric 5 # modem metric is typically 3, but put it really low just in case
fi