cf. Сайт https://nextcloud.com не открывается
It is not entirely clear. When I was in Russia earlier this year, sometimes it worked (DSL or whatever it is), sometimes it did not (MTS iirc), and different with WiFi providers. So it seemed that depending on the provider one of the hops inbetween was blocked.
From Roskomnadzor I got the reply to change the IP, which is ridiculous. However, this will happen in the not so far distance anyway as we plan to move the website to another hoster. That’ll probably help. Otherwise, Roskomnadzor running Amok and banning half the IPs is just insane.