Sophos XGS 116w - DNS Request Route

A customer site has a 2nd gateway that is required to access one of their vendor's systems. Our Sophos XGS has static routes in place to direct any traffic intended for the vendor network to the 2nd router. Rather than adding host entries for the vendor in the DNS config I'm trying to set up a DNS request route that points to the vendors DNS servers, thus future proofing the setup against any IP address changes on the vendor's end.

Unfortunately its not resolving the URLs. I've added the base domain to the DNS request route section and the target DNS servers which I had set up in the hosts section.