Initial script by https://blog.harrier.us/reconnecting-a-failed-vpn-connection-on-synology-dsm-6/

check_vpn.sh

#!/usr/bin/env bash
#===============================================================================
#         FILE:  ReconnectSynologyVPN.sh
#
#  DESCRIPTION:  Reconnect a disconnected VPN session on Synology DSM
#    SOURCE(S):  https://forum.synology.com/enu/viewtopic.php?f=241&t=65444
#
#       AUTHOR:  Ian Harrier
#      VERSION:  1.0.0
#      LICENSE:  MIT License
#===============================================================================

#-------------------------------------------------------------------------------
#  Pull in VPN config files
#-------------------------------------------------------------------------------

if [[ -f /usr/syno/etc/synovpnclient/l2tp/l2tpclient.conf ]]; then
  L2TP_CONFIG=$(cat /usr/syno/etc/synovpnclient/l2tp/l2tpclient.conf)
else
  L2TP_CONFIG=""
fi

if [[ -f /usr/syno/etc/synovpnclient/openvpn/ovpnclient.conf ]]; then
  OPENVPN_CONFIG=$(cat /usr/syno/etc/synovpnclient/openvpn/ovpnclient.conf)
else
  OPENVPN_CONFIG=""
fi

if [[ -f /usr/syno/etc/synovpnclient/pptp/pptpclient.conf ]]; then
  PPTP_CONFIG=$(cat /usr/syno/etc/synovpnclient/pptp/pptpclient.conf)
else
  PPTP_CONFIG=""
fi

#-------------------------------------------------------------------------------
#  Process config files
#-------------------------------------------------------------------------------

# Concatenate the config files
CONFIGS_ALL="$L2TP_CONFIG $OPENVPN_CONFIG $PPTP_CONFIG"

# How many VPN profiles are there?
CONFIGS_QTY=$(echo "$CONFIGS_ALL" | grep -e '\[l' -e '\[o' -e '\[p' | wc -l)

# Only proceed if there is 1 VPN profile
if [[ $CONFIGS_QTY -eq 1 ]]; then
  ##/usr/syno/bin/synologset1 sys info 0x11100000 "check-vpn: There is 1 VPN profile. Continuing..."
  echo "[I] There is 1 VPN profile. Continuing..."
elif [[ $CONFIGS_QTY -gt 1 ]]; then
  /usr/syno/bin/synologset1 sys err 0x11100000 "check-vpn: There are $CONFIGS_QTY VPN profiles. This script supports only 1 VPN profile. Exiting..."
  echo "[E] There are $CONFIGS_QTY VPN profiles. This script supports only 1 VPN profile. Exiting..."
  exit 1
else
  /usr/syno/bin/synologset1 sys warn 0x11100000 "check-vpn: There are 0 VPN profiles. Please create a VPN profile. Exiting..."
  echo "[W] There are 0 VPN profiles. Please create a VPN profile. Exiting..."
  exit 1
fi

#-------------------------------------------------------------------------------
#  Set variables
#-------------------------------------------------------------------------------

PROFILE_ID=$(echo $CONFIGS_ALL | cut -d "[" -f2 | cut -d "]" -f1)
PROFILE_NAME=$(echo "$CONFIGS_ALL" | grep -oP "conf_name=+\K\w+")
PROFILE_RECONNECT=$(echo "$CONFIGS_ALL" | grep -oP "reconnect=+\K\w+")

if [[ $(echo "$CONFIGS_ALL" | grep '\[l') ]]; then
  PROFILE_PROTOCOL="l2tp"
elif [[ $(echo "$CONFIGS_ALL" | grep '\[o') ]]; then
  PROFILE_PROTOCOL="openvpn"
elif [[ $(echo "$CONFIGS_ALL" | grep '\[p') ]]; then
  PROFILE_PROTOCOL="pptp"
fi

#-------------------------------------------------------------------------------
#  Check the VPN connection
#-------------------------------------------------------------------------------

if [[ $(/usr/syno/bin/synovpnc get_conn | grep Uptime) ]] && [[ $(ping -c 1 google.com) ]]; then
  /usr/syno/bin/synologset1 sys info 0x11100000 "check-vpn: VPN is already connected and retrieving data. Exiting..."
  echo "[I] VPN is already connected and retrieving data. Exiting..."
  exit 0
fi

if [[ $PROFILE_RECONNECT != "yes" ]]; then
  /usr/syno/bin/synologset1 sys warn 0x11100000 "check-vpn: VPN is not connected, but reconnect is disabled. Please enable reconnect for for the \"$PROFILE_NAME\" VPN profile. Exiting..."
  echo "[W] VPN is not connected, but reconnect is disabled. Please enable reconnect for for the \"$PROFILE_NAME\" VPN profile. Exiting..."
  exit 1
else
  /usr/syno/bin/synologset1 sys warn 0x11100000 "check-vpn: VPN is not connected or is not retrieving data. Attempting to reconnect..."
  echo "[W] VPN is not connected or is not retrieving data. Attempting to reconnect..."
  /var/packages/DownloadStation/scripts/start-stop-status stop
fi

#-------------------------------------------------------------------------------
#  Reconnect the VPN connection
#-------------------------------------------------------------------------------

/usr/syno/bin/synovpnc kill_client --name=$PROFILE_NAME
sleep 20
echo conf_id=$PROFILE_ID > /usr/syno/etc/synovpnclient/vpnc_connecting
echo conf_name=$PROFILE_NAME >> /usr/syno/etc/synovpnclient/vpnc_connecting
echo proto=$PROFILE_PROTOCOL >> /usr/syno/etc/synovpnclient/vpnc_connecting
#/usr/syno/bin/synovpnc reconnect --protocol=$PROFILE_PROTOCOL --name=$PROFILE_NAME --retry=1 --interval=30
/usr/syno/bin/synovpnc connect --id=$PROFILE_ID
sleep 20

#-------------------------------------------------------------------------------
#  Re-check the VPN connection
#-------------------------------------------------------------------------------

if [[ $(/usr/syno/bin/synovpnc get_conn | grep Uptime) ]] && [[ $(ping -c 1 google.com) ]]; then
  /usr/syno/bin/synologset1 sys info 0x11100000 "check-vpn: VPN successfully reconnected. Exiting..."
  echo "[I] VPN successfully reconnected. Exiting..."
  /volume2/homes/tommy/Scripts/port_forwarding.sh
  ##The file above can be found here (remove if not required); https://gist.github.com/GamerGun/20f08da8ff2712db6e62c5edc83fd541
  exit 1
else  
  /usr/syno/bin/synologset1 sys err 0x11100000 "check-vpn: VPN failed to reconnect. Exiting..."
  echo "[E] VPN failed to reconnect. Exiting..."
  exit 1
fi

Really nice script. I had to modify it a little for the Synology router because it only has the ash (not bash) shell. For example, it does not have the grep -P flag.