Connecting my desktop to the mobile network via wifi tethering the pages load fine. So yes, this should be router or ISP related. I don't think that my router filters webpages, I haven't set up such firewall rules. So I will call my ISP.
Strange thing is that Firefox opens some of these pages even on my ISP's network. Some pages fail on Firefox, too. Chromium, Vivaldi, Brave fail to open all of them.
If I connect my mobile via wifi to the same LAN and turn off mobile data usage to ensure that I am routed to the same ISP the pages still load fine on my mobile. I don't really get it.