I am using the DoH service of Mullvad (in the Android settings, choose Settings, Network and Internet, Advanced, Private DNS), using their adblocking DNS under adblock.doh.mullvad.net. Since a few days however, my phone says “Couldn’t connect” and there is no internet connection, neither with wifi nor with mobile data. The same is true for their other address doh.mullvad.net, where they do not block ads. Interestingly though, my home router still connects to their DNS. And adding another private DNS like one.one.one.one in my phone also works.
I am using e_star2lte-userdebug 10 QQ3A.200805.001 eng.root.20220905.192741 dev-keys,stable-release, i.e. the current 1.3 stable on my Galaxy S9+.
I have observed something similar for Mullvad using either the domain names or the IP adresses.
Upon changing the DNS (names or IP), everything is working ok, after some time (hours or overnight), I experience networking errors, websites are not being found, email connections are not functional, and account syncing with ecloud is not working.
Everythings starts working again, after switching to Quad9 IP.
In addition, I found this to also occur when using the IP addresses of digitalcourage.de
That is on FP3+ running /e/OS 1.3 (1.3-20220905, happened also for 1.0 and 1.2.
The problem persisted all through version 1.19.1 (including that version). When I reset all app preferences everything worked again. That is, I used the option
Settings > Apps and notifications > See all [# of apps installed] apps > Reset app preferences (that option is hidden in the three-dots menu in the upper left corner).
After that, I could use secure DNS from Mullvad again. (Although note that in the meantime they changed the URL to base.dns.mullvad.net. I tried the new URL when they changed it, of course.)