PCs connect to internet but fail to load web sites

Apologies in advance for a rather long post, but the details are important. I have a network problem which has so far defied all attempts to solve it, even with the help of Google and Gemini! It relates to our Village Hall within which we have recently (four months)changed broadband provider from PlusNet to Zen Internet. The latter provided a Fritz!Box router as part of the installation. The hall has three wifi repeaters installed within ceilings which share a common SSID and the Fritz!Box provides another connection with its own SSID. Most people have no problem connecting to the internet wit

Question by Phil Tilson on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.