I have wasted the last 2.5 hours trying to see where I went wrong with my configuration and I just can't.
For the record, I am running OpenSuse Tumbleweed with Gnome, latest update for everything. Up to now I have been using AdGuard as my DNS resolver, but am now trying to switch to Mullvad but at this point I think I probably don't want to anymore. Reason being, I just can not get it to work for the life of me.
My system has NetworkManager installed so I go there, select my connected Wifi, and enter Mullvad's DNS address 194.242.2.4 in thr IPv4 section, then I go to check to see if it shows I am using their DNS and it Firefox AND Vivaldi give no internet connection errors. I go back to Adguard DNS and my internet is back working again. I go back to Mullvad, you guessed it, no internet once again. I even tried Cloudflare and Quad 9's DNS addresses and both of those worked as well but Mullvad's just does not want to work and I am going insane over it.
And no I can not edit resolv.conf through the terminal because NetworkManager will override it and no I don't want to delete NetworkManager. Any feedback would be appreciated.
Edit: I have Mullvad DNS on my phone and got it running with zero issues so this is more of a Linux problem than a Mullvad DNS problem I think.
Solution:
Open terminal and follow through
sudo zypper install systemd-network
sudo nano /etc/systemd/resolved.conf
Copy paste this into the file that you just opened and change the DNS to whichever DNS provider you are using.
[Resolve]
DNS=194.242.2.4 2a07:e340::4
FallbackDNS=194.242.2.2 2a07:e340::2
Domains=~.
DNSSEC=yes
DNSOverTLS=opportunistic
#MulticastDNS=no
#LLMNR=no
#Cache=yes
#CacheFromLocalhost=no
#DNSStubListener=no
#DNSStubListenerExtra=
#ReadEtcHosts=yes
#ResolveUnicastSingleLabel=no
Ctrl + O to write out and Ctrl + X to exit back to the terminal main page.
ln -sf ../run/systemd/resolve/stub-resolv.conf /etc/resolv.conf
sudo systemctl start systemd-resolved
sudo systemctl enable systemd-resolved
sudo systemctl restart NetworkManager
Boom it should be working now.
Mullvad (apparenlty, first time I've heard from the service) uses DNS over TLS and I don't think that the current GUI version has the option to enable it. Here's a quickly googled howto from Fedora on how to enable it on your system. If that doesn't help search for 'NetworkManager DOT' or 'DNS over TLS'.
Right on the money: See also the official mullvad docs: https://mullvad.net/en/help/dns-over-https-and-dns-over-tls#linux
The solution on their page does not work as my network settings are controlled by Network Manager
I tried the guide you sent, and it gives me an error in the terminal when I try to restart NetworkManager. This is caused by the thing in step 2, and when I remove the file that was created in step 2, NetworkManager starts fine again, but now I need to have a DNS IP address entered into IPv4 and IPv6 per network connection, and it can not be Mullvad's DNS servers otherwise I again get no connection which again just puts me back at square one, only now I have a systemctl command running in the background for no reason.