No, I am sorry, I confused you by first trying 184.108.40.206 and seeing it was not working, then I tried just the DNS of Mullvad (not their VPN), and found out later I can't use that.
Now I tried with Quad9 and with the website whoismydns I finally see that my DNS changed! I just had to look from another clean browser, my fault.
I used QoS in previous versions of OpenWrt - I currently don't need QoS, SQM, etc. so I'm not familiar. Security-wise, the default OpenWrt and settings are good.
The only change from default I perform is the following:
All firewall settings that say "Reject" - I change to "Drop"
Please understand the technical implications of this change.
Nope, I don't add generally consider adding packages to make things more secure or perform better. I can't think of any, except that if you do have a Mullvad account, you can install the VPN packages necessary to configure/use on the router - setup clients to route thru it, etc.
I believe DoH/DoT was also suggested in another post.
Hope this helps and congratulations on solving your issue.
I tried with 220.127.116.11 (which is the DNS I set up in OpenWRT) and I get:
root@OpenWrt:/# ip route get 18.104.22.168 from 192.168.93.1
22.214.171.124 from 192.168.93.1 via 192.168.1.1 dev wan
It's the same output if I use 126.96.36.199 or 188.8.131.52, just the first part is changing obv. From what I am understanding it just says that the route goes to the main router (192.168.1.1) as it should be, right?
Regarding the solution, my topic wasn't about DNS in particular but more general setup, including DNS. I will certainly mark a post as solution later on, I was just experimenting with the nextdns package and then I realized I would like to try a more general dns-over-https or dnscrypt-proxy package. I see with that one I can also use NextDNS among others. I would like to set up my DNS not with IPv4 (like we just succeded, it was the first step) but protecting my DNS queries with one of the above packages, for instance.