Unexpected DNS behavior

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I am running Asuswrt-Merlin version 384.18, with two VPN clients running at the same time, using Policy Rules to assign devices to each.

I also have Enable DNS-based Filtering set to On, with Global Filter Mode set to Router.

When I set a device's IP to go via WAN interface by Manually Assigned IP under LAN -> DHCP Server, and also set its DNS server to be an external DNS server,, I get an unexpected result: when I do a DNS leak test on the device, it gives me my WAN/ISP's IP as expected, but DNS IP is from VPN client 2 instead of anything related to

The only way I can get to be used on the device is to add it to the Client List under DNSFilter and set its Filter Mode to a Custom that I have set as

Has anyone got any ideas why (a) just setting it to under LAN -> DHCP Server manual assignment is not enough, and (b) how/why it uses VPN client 2 if I don't use the Custom setting in DNSFilter? That just seems totally random!

Not sure if it is relevant, but under VPN client 1 I have added the router itself to use the WAN interface as I have seen that in this forum as being a recommended setting.

Any help would be much appreciated.

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