I suspect this topic is accumulating various kinds of DNS problems.
Note that the original post is for Turris 1.x HW which has never been “distributed normally” (just lent as part of a security research project in CZ and later allowed to be kept by participants). A significant DNS difference of 1.x is that it uses Unbound whereas all other Turrises use Knot Resolver (by default). I don’t know Unbound too well.
Generally, if you have DNS issues, I’d personally first try other forwarding setting than the default. (see (re)Foris UI, DNS tab) I think it’s not rare to encounter ISP servers that have issues in edge cases when obtaining DNSSEC records. My favorite mode is without forwarding, but forwarding to anyone from the list should also be reliable (and it’s even better in case ISP intercepts DNS).