@Mlipka I'm afraid, joy about it lasted only for a day. I had to restart to router twice this morning. Haven't changed the DNS settings in Opera, neither on desktop nor on Android and never used VPN.
Still, I wonder if this is just coincidence, but I'd rather use Firefox instead for a while in order to exclude it wasn't any general issue in the router. So if it's going well for another week, I'll try to install Opera again. But for now it's really annoying, if Opera really is the suspect and behaving strangely.
Some would say it could also be an issue with Flow, I don't know, actually. But if it's really Opera-related, it only appeared after Version 128 not before. There were hardly any issues before, only some smaller problems which disappeared after restarting the router, but they didn't prevail.
I'd love to use Opera as my default with Firefox as back up, but for now I'll keep using the latter exclusively.