OpenVPN Client config and DNS Leak

I’ve done a great deal of search within this forum and elsewhere and I hardly find a solution to plug the DNS Leak.
The whole idea is to setup the OpenVPN client and run it so that my client devices when the tunnel is active all the traffic goes via the tunnel as well as the DNS requests.
I’ve thoroughly followed the @omni guide VPN Client Settings get openVPN to work with any Provider and some other insights on this forum but none can address the DNS Leak.
I used to forward the upstream DNS request to but even if I uncheck the DNS Forwarding for whatever reason Cloudflare still resolves the DNS requests.
I’ve created the necessary firewall rule and still doesn’t address the problem.
Is the root cause sitting under the knot-resolver, the firewall or elsewhere?
My VPN Provider (ExpressVPN) has no documentation on howto setup OpenWRT routers.
Any help is much appreciated.

Warm regards

Device: Turris Omnia 2020, TO 5.1.9 HBS