bubbleshoot Even the native Mullvad app won't connect, not even without a VPN app.
I am not following here. I think Mullvad is a VPN app, so I don't understand how the device could simultaneously have Mullvad and be "without a VPN app".
bubbleshoot I created another user and the internet is working normally there.
Then the problem very likely is a VPN or DNS configuration in the owner profile. That is consistent with the network having worked in the owner profile during the time between the factory reset and the installation of RethinkDNS.
What happens if a factory reset is done and Mullvad is installed instead of RethinkDNS?
Does the network blockage in the owner profile happen only on a home network, or does it also happen when using a public Wi-Fi such as a library or coffee shop?
Have other settings been changed, such as disabling connectivity checks?