Test router is connected to main KN-1811 ("Titan"), 'Cloud-based content filtering and ad blocking' is not installed, remove all DNS except provider one, configure DoH Google - Stubby is not able to bootstrap, fails immediately according to the logs. Add 1.1.1.1 as a common DNS. Use for some time (it works, able to bootstrap, able to resolve using DoH - at least DNS leak test confirms Google DNS as an upstream), after it starts to create issues as above.
Will add self-test at evening.