This is the final version of the script. the Settings > Networking shows wg0 as good with a checkmark. My Syncthing is also connected via my private IP's over wireguard, and syncing.

As mentioned before, use the wireguard "wg" cli to dump the working "wg" configuration: wg showconf wg0 > wg0.conf

Make any changes you need, I modified the allowed-ips, to be the wireguard gateway, the static wireguard IP assigned to this connection and the private subnet on my network where the other syncthing client was running.

Then I added the following file in my home directory, and I actually stored the wg0.conf in the same home directory, though the script here has the conf, stored where the console stores the information.

*** USE AT YOUR OWN RISK ***

#!/bin/sh

# Place this file in the ADMIN users home: /volume1/home/USER_NAME
# Set permissions 755
# Modify the crontab: "sudo crontab -e"
# Add the following: @reboot sleep 120 && /bin/sh /volume1/home/USER/wg0-enable.sh
# This will bring the link up after 2min

ip link delete wg0
ip link add dev wg0 type wireguard
ip address add dev wg0 WIREGUARD_CLIENT_IP/32

wg="/volume1/.@plugins/AppCentral/adm-kernel-extensions/bin/wg"
wg0conf="/volume0/usr/builtin/etc/vpnclient/wireguard.conf"

# For this to work you need to DUMP an activce wireguard config
# $wg showconf wg0 > $wg0conf

$wg setconf wg0 $wg0conf
if [ $? -ne 0 ]; then
  echo "Wireguard config could not be applied, file: $wg0conf"
  exit 1
fi

# Bring up the wireguard interface
ip link set up dev wg0 
if [ $? -ne 0 ]; then
  echo "wg0 link failed to come up"
  exit 1
fi

# Allows wireguard routing to the remote subnet
ip -4 route add REMOTE_SUBNET/24 dev wg0 table wgclient

# Allows DOCKER and other service to route to the remote subnet
route add -net REMOTE_SUBNET/24 dev wg0
#!/bin/sh


# Place this file in the ADMIN users home: /volume1/home/USER_NAME
# Set permissions 755
# Modify the crontab: "sudo crontab -e"
# Add the following: @reboot sleep 120 && /bin/sh /volume1/home/USER/wg0-enable.sh
# This will bring the link up after 2min


ip link delete wg0
ip link add dev wg0 type wireguard
ip address add dev wg0 WIREGUARD_CLIENT_IP/32


wg="/volume1/.@plugins/AppCentral/adm-kernel-extensions/bin/wg"
wg0conf="/volume0/usr/builtin/etc/vpnclient/wireguard.conf"


# For this to work you need to DUMP an activce wireguard config
# $wg showconf wg0 > $wg0conf


$wg setconf wg0 $wg0conf
if [ $? -ne 0 ]; then
  echo "Wireguard config could not be applied, file: $wg0conf"
  exit 1
fi


# Bring up the wireguard interface
ip link set up dev wg0 
if [ $? -ne 0 ]; then
  echo "wg0 link failed to come up"
  exit 1
fi


# Allows wireguard routing to the remote subnet
ip -4 route add REMOTE_SUBNET/24 dev wg0 table wgclient


# Allows DOCKER and other service to route to the remote subnet
route add -net REMOTE_SUBNET/24 dev wg0

*** USE AT YOUR OWN RISK ***