Nick, You can also do a traceroute from the PAN to the DNS server IP address and check the different routers it goes through. One more option is to use service route. You can go to the device tab---->setup on the right frame bottom you will get the option for "service route configuration" you can choose the interface which is connected to the internet as being the source interface for the DNS query. Please find a snapshot for that page when you can set this up.
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