AdGuardHome Upstream DNS Server with AirVPN (DNS Leak)

[zhyp 0]

Hello, I've recently moved from Mullvad to Air VPN, and with my current config I use AdGuard Home for DNS blocking, and I used Mullvad DNS servers with it:

https://doh.mullvad.net/dns-query
tls://doh.mullvad.net

But now, after switching to Air VPN I didn't find it anywhere Air VPN DNS Servers, so I believe that they don't have any.
Anyone versed on this, what would you recommend, for me to ditch AdGuard Home altogether (which I don't really like, I would like to keep it)? Or there's no actual problem in using with the old VPN DNS Servers?
I did see that Air VPN has a DNS Blocking solution, but since I already spent the time configuring this beforehand, I'm not so happy with just ditching it.
This configuration is done entirely on my router, this is my DNS Servers when 'ipconfig /all' is run:

fd5e:8689:2708::1 //router ipv6
10.128.0.1 // air vpn
192.168.2.1 // router ipv4

And all the servers that are shown on ipleak.net are the VPN and Mullvad's Servers.
Even though this is not an issue with Air VPN itself, I would like to know if someone using this VPN has experienced this somehow, thanks.

Edit1:
It looks like the DNS 'Leak' is coming from the IPV6 Devices only, since they don't have the first DNS Server to be the VPN one, like it's on IPV4. They try to use both IPV4 and IPV6 DNS Servers at the same time, causing the leak on the IPV6 server (that is using my router). Unfortunately this sounds like there are no solutions where I keep AdGuard Home, I'll keep trying and see where this leads me. Edited ... by zhyp

[Air4141841 23]

https://airvpn.org/specs/.  10.4.0.1. lists the dns servers for openvpn

I turn off ip6 at the router level to minimize issues 

[zhyp 0]

I did some extra research after posting this, and I did think about that. The only problem is that since my AdGuard Home gives DNS for all my devices and this is a private DNS Server, they would just fail if not tunneling into the VPN.
For now, I disabled IPV6 DNS since it looks like the browser or windows prefers it over the IPV4. And I don't know how to 'overwrite' it on the router level (without messing with windows, and just letting DHCP do its thing).
Basically, currently I'm sending both private (10.4.0.1) and my router (192.168.2.1) as DNS Servers, with the private one being preferred on the device, and most of the time it'll hit the private one, but I'm not sure why. If I just remove my router as the DNS Server, I would need to go manually and remove the DNS from my host and reconnect, which would be annoying.
Do you use OpenWrt too? I'm using it with PBR for some extra mobility when needing to disable VPN on devices quickly, that's why all the fuss, otherwise I would just remove my router as a DNS option.