Advertise router DNS to VPN clients as 2nd DNS server

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Hi all, I've got a TUN OpenVPN Server running fine on my RT-AC86U with Merlin 384.13.
But I have a problem when specifying the DNS servers my VPN clients should use.

For example: router IP is, primary DNS server is
I want my VPN clients to use the primary DNS server (which isn't the router) by default, but fall back to the router.

For example, I put this in custom configuration:

push "dhcp-option DNS"
push "dhcp-option DNS"

However, now the router won't respond to DNS queries.

When I check "Advertise DNS to clients", then the router will reply to DNS queries, but the result is that it will also add another push entry to the OpenVPN server conf. The result is:

push "dhcp-option DNS"
push "dhcp-option DNS"
push "dhcp-option DNS"

So now my router will be used as primary DNS, but I need to use it as a fall back DNS server when the primary one is offline.

Since release 384.5 (13-May-2018), the option to respond to dns queries has been removed. But I think that option, to only respond to DNS queries without also adding the push entry to the server conf, would solve my problem...

Does anyone have a suggestion how I can solve this? Changing the client config isn't an option.

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