Reconnect scripts location:
/usr/syno/etc.defaults/synovpnclient/scripts/ip-down
/usr/syno/etc/synovpnclient/scripts/synovpnclient.sh
Edit NAS /etc/crontab
0,5,10,15,20,25,30,35,40,45,50,55 * * * * root /bin/bash /usr/syno/etc/synovpnclient/scripts/synovpnclient.sh start
Reference: Manfred, VPN client auto reconnect not reliable, Jan 14, 2021, https://community.synology.com/enu/forum/1/post/138326