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You need to do three steps:
Add a route to the VPN server via your physical gateway so that VPN tunnel packets can reach the VPN server
ip route add <VPN server IP address> via 192.168.68.1
Delete existing default route
ip route delete default via 192.168.68.1
Add default route via the tunnel
ip route add default dev tun0
However, if the VPN server ...
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