Destination Port Unreachable

You need to add tun0 to the wan zone as a device not a network.
list device 'tun0'
Or you can do it the old way and create a dummy network to associate a network name with the device.
(in /etc/config/network)

config interface 'vpn'`
    option device 'tun0'
    option proto 'none'

then add 'vpn' to the list of networks in the wan zone.